The modified BDU’s on this page, often called “RAID mods” were relatively common in the early 2000’s in the early War on Terror period due to supply issues. While these are associated with Special Forces, many “regular” units used them as well until suitable supplies could be manufactured. These uniforms often had body pockets moved to the arms and velcro added for patches and rank insignia. The arm pockets are possibly for easier access to medical supplies when treating casualties, since it unlikely for wounded soldiers to land on their sides, and lower pocket removal makes it easier to tuck into pants to minimize noise (source: https://lchshistory.pastperfectonline.com/webobject/66822BB5-BD09-4720-A04A-897154625540).
The modifications listed below do not, and cannot possibly, include every single possible variation since many of these are custom-tailored items. A partial list with relative frequencies can be seen here: https://oefoif.forumotion.net/t3248-modified-bdu-and-dcu-jackets
Sample 1:
No buttons on pockets. No longer in collection.
Velcro:
Tags:
Sample 2:
Also no longer in collection.
Sample 3:
No longer in collection. Similar pocket and velcro arrangement to khaki BDU (Sample 8, below).
Sample 4:
No longer in collection.
Tags:
Velcro:
Sample 5:
Check pockets are lower pockets moved up and slanted.
Velcro:
Tags:
Sample 6:
Pockets:
Arm pockets. 
Chest pockets.
Back Velcro:
Tags:
Sample 7:
Pockets slanted outwards, perhaps for easier access while wearing under plate carrier.
Patches:
Collar insignia is for Civil Engineering Corps Lieutenant.
Tag:
Arm Pockets:
Sample 8:
Similar pocket and velcro arrangement to woodland BDU (Sample 3, above).
Small square piece of velcro behind collar.
Pockets:
Tags:
Sample 9:
NSW type modifications, same velcro and pocket arrangement as Sample 10. Found together with Sample 10.
Tags:
Velcro:
Sample 10:
NSW type modifications, similar velcro and pocket arrangement as Sample 9. Found together with Sample 9. Appears to be totally unused.
Arm Pockets:
Chest Pockets:
Extra loop area under branch tape for rating patch? 
Tag:
Sample 11:
No velcro. Arm pockets stitched by hand in the field.
Pockets:
Arm pockets. 
Chest pockets.
Chest pockets are unmoved, but arm pockets are lower pockets moved and hand-stitched.
Patch Remnants:
Sample 12:
Ranger style velcro arrangement.
Tags:
This modification was done at the factory since the item is new (still has hang tag) and stitching for velcro is not visible from the outside.
Handwritten NSN 8415-01-162-6766 (assigned October 25, 1980) predates the printed 8415-01-390-8544 one (assigned May 27, 1994) that was crossed out.
Arm Pockets:
Usage Photo:
Sample 13:
Standard twill woodland BDU, except with strobe pouch attached to left arm. Likely associated with Navy, maybe Naval Special Warfare, since it was found at a swap meet with other Navy BDU’s with varying name tapes.
Arm Strobe Pouch:
Strobe pouch is sewn onto right arm.
Patch Remnant:
Circular shadow on upper left pocket. 
Left arm does not appear to have any patch remnants.
Circular patch ghost on left upper pocket.
Tags:
Usage Photo:
Sample 14:
Lower pockets removed, upper chest pockets remain intact in normal orientation, no arm pockets. Shoulder patches include Ranger Battalion scroll, airborne tab, and 504th Military Intelligence Brigade. 504th and Airborne patches were attached with adhesive after Ranger scroll was removed (adhesive residue covers Ranger scroll holes). Chest patches include Expert Infantryman and Jump Wings.
Sample 15:
Probably a souvenir shirt made from a modified DCU in the style of a Crye Combat Shirt. Velcro and pocket arrangement is very similar to NSW types above (Samples 9 and 10).
Jacket that was further modified was probably heavily used on operations.
Arm Pockets:
Left arm pocket has pen pocket inside. 
Inside:
Sleeve/shoulder area DCU material is sewn directly on top of intact drifire shirt.
Sample 16: Black Cotton Set
Jacket and trousers came together in a set. The material is cotton, not 50/50 NYCO, and this would be a non-Berry Compliant private purchase item since it was assembled in the Dominican Republic.
Jacket and trousers are both 1998 dated.
Chest pockets are made from lower waist pockets, slanted inwards.
Arm pockets are made from former chest pockets and are sewn perpendicular to the shoulder (“straight”). They have velcro loop covering the entire exterior, and close with a velcro strip that takes significant effort to open.
There are 2 flag-sized panels, one on the shoulder above the pocket where the flag normally should be, and the other on top of the shoulders.
Trousers:
Trousers are not modified.
Pants are also dated 1998.
Modified black BDU’s were used by SEALs in Central America in the 1980’s – 1990’s (source: https://iacmc.forumotion.com/t6537-black-modified-bdu-set-seal-used).
Sample 17: Six Color Desert
Lightly modified with only velcro for patches on chest and shoulders. Chocolate Chip DBDU jackets are rarely modified compared to their woodland and DCU counterparts.
Velcro:
Green velcro on arm for flag patch; tan on chest for name or other insignia.
Tags:
1990 date. This is a later production example since there is no internal “lining”.
Sample 18: Accuracy 1st Combat Shirt
Modified to combat shirt like Sample 15.
Accuracy 1st was incorporated in 2005 (https://opencorporates.com/companies/us_tx/0800495293) and is involved in sniper training. They are still active at https://www.accuracy1st.com/.
Arm Pockets:
Inside:
BDU material sewn over outside of back to provide structural support. Arms were cut off and BDU ones sewn in.
Sample 19:
Earlier six pocket modification without velcro that resembles a Soviet Afghanka. This was probably meant to be worn with a webbing harness, not a vest or plate carrier because all 4 pockets were left on the front.
Tags:
White 1981 (first year of issue) tags.
Arm Pockets:
Remnants of Army Special Forces patch and tabs on sleeve. The pockets were a different Air Force or Navy BDU’s upper chest pockets.
Chest Patches:
Numbers:
4-digit number written on inside with sharpie.
Cuff:
Many woodland BDU jackets up to the early 1990’s have small, separate cuff adjustment tabs. Later BDU’s have it integrated into the cuff piece.