What does the hydration-compatibility of backpack bag means? ?

Question

I am going to buy a light-weight backpack bak for travel, and came across bag which is hydration-compatible. I don't know what it means. Would you explain it to me? Thanks.

Answer 1

there is room in the bag for a hydration unit

see the link

very useful, saves effort of getting bottles out for drinks

http://www.millets.co.uk/Walking-Kit/Hyd...