Base amount is also called an expense stop. It is typically used for items such as property taxes or insurance where the current amount is known, but there could be significant increases in the future, and the landlord does not want to be responsible for them.
In the recovery setup at the tenant level, you specify a base amount for each detail line. The tenant share is then calculated on the amount in the pool that exceeds the base amount.
For example, the base amount is $10,000. The total expenses for the year are $12,000. The tenant's prorata share is 10%.
The calculation would be: (total expenses - base amount) * prorata share, in this case, ($12,000 - $10,000) * 10% = $200.
If the base amount is close to what you expect the expenses will be, you can set the estimate charge to zero (0). Then, if the actual expenses exceed the base, the difference is made up in the reconciliation charge.