"InSummary" is a check box setting on Manage Company Databases and Manage Budgets.
In order for a BudgetID within its CompanyDB to be available for viewing in the Budget Summary of the Accounts Details tab, both its related CompanyDB and BudgetID checkboxes needs to be checked.
When both boxes are checked for any given BudgetID the Spread Method (see C. in this linked article) dropdown menu makes available the spread according to the selected BudgetID. By selecting the BudgetID the current Account Number (Budget Account Number) you are spreading will spread according to the spread of the selected BudgetID's Account Number.
Please note it is a useful tool to limit the number of available Company DB and BudgetID to only those that are necessary by unchecking the "InSummary" boxes as this can result in a "decluttered" Budget Summary view.
In the below example, 2016 Actual, 2016 test and FY15 Budget were selected.
And below is the result from the above selection; only the selected BudgetIDs are in the view.