The allocate on checkout feature prompts you to allocate specific items to a reservation when the stage is progressed from Confirmed to Checked out.
GBAL is an optional (paid) feature in BRM. This feature is only relevant to users with that option.
First, an Admin user must enable this feature in Settings > Admin > Features and Customizations.
Once enabled, when a reservation with generics is checked out, a popup appears giving you the choice of available items from each Product Line (PL).
Select the item (e.g. actual bike) you wish to assign from each dropdown and click allocate to proceed. Alternatively, you can choose to skip allocation.