ShowForRunIn is a sandbox setting in Sandboxie Ini. It specifies whether Sandboxie will show this box in "run in box" selection prompt.
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[DefaultBox]
ShowForRunIn=n
Specifying n indicates that this sandbox will not be shown as a candidate in "run in sandbox" selection window.
Related Sandboxie Plus setting: Sandbox Options > General Options > Box Options > Show this box in the 'run in box' selection prompt