This a sub-tab on the Account Details tab.  Please see Budgeting Dashboard, Basic Budget Entry, Custom Budget Entry, Custom Reports, or Custom Reports with Line Item Details articles on how to use the functionality on this sub-tab.

Please note that when a Account Number (Budget Account Number) has been designated as "Payroll", "AllocAcct" or "ContraAlloc" the Assumptions Sub-tab is respectively replaced with a Payroll Projections Sub-tab or a Allocations Sub-tab.  As these Sub-tabs are non-editable and read-only the Adjustment Sub-tab can be used to enter additional budget values and/or to adjust numbers.

Please note that some functionality and display for this Sub-Tab can be customized on App Config/Menu Editor.  Please see the Application Configuration section item 2 of this article for more details.

Related Article:

How Do I Manually Adjust Payroll Amounts on the Account Details tab?