It has 15 inventory slots which hold NO predefined items. Player has to manually edit the config file and place item ID's for this backpack to be useful in any way
NOTE: To change which items go into Adventurer's Backpack, locate Forestry configuration folder and edit the file backpack.conf with item ID's of your choosing.
It seems this backpack must be setup manually.
https://ftbwiki.org/Adventurer%27s_Backpack