"SubsessionEvaluateCells" is a front end token that interrupts the current evaluation, evaluates the selected cells in a kernel subsession, and resumes the main evaluation.
This token is equivalent to the menu command Kernel Evaluation Evaluate in Subsession.
This can be useful for checking on the status of variables in a long calculation or for debugging. See Dialog for more information.
To execute a front end token, you typically evaluate a command of the form FrontEndExecute[FrontEndToken[token]].