There is an optional feature called soft booking which can be used to create tentative allocations.
Note that tentative allocation values are not "summed" on top of the confirmed allocations. When viewing "Tentative & Confirmed" the allocations boxes show tentative values if there are tentative allocations in that box, otherwise the box will show confirmed allocations. So the tentative and confirmed values are not added together in the box. Same applies on the "Confirmed & Tentative" view type.
The soft booking functionality is not enabled by default. If you are interested, it can be enabled by sending a request to firstname.lastname@example.org