can some one please explain how the adjustable travel system works on rock shox forks please. they are the rockshox reba team u turn model. travel 90 – 120mm

"U" turn a knob, & the travel increases/decreases. 😉

Coil versions via compressing a spring, Air by moving a damper position.
(I think).

On the coil, there's a chunky plastic screw thread inside the bottom of the coil spring. Winding the adjuster turns the spring which draws the lowers up as the plastic part is attached to the lower.

In an air uturn, the position of the top of the air chamber is moved up and down – again a plastic part inside the airtube inside the leg – altering the length of the fork.

which adjusts the positioning/geomotry of the bike aswell i guess.

so best of both worlds almost,

90 mm for going up with min bounce, 120 for the bumpy downs?

Yeah. If you can be bothered to stop and adjust it's great.

Wouldn't do with out it myself and you don't need to stop…can be done on the fly. Do prefer the Fox Talas though as the travel adjust is so much quicker than the slightly more laborious dialling up/down of the U-Turn.