Save a variable from an User Task and use as condition variable

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At my user task I am filling some data(which won't be stored in ACT_RU_VARIABLE) and as soon as I click the submit button, i am invoking taskService.complete(). Now lets say if I want to use, one field which I fill at the user task, as a condition variable. How do i do that assuming we are using only activiti's 28 tables?

example: Lets say we have a leaveRequest, the leave is approved if its less than 3 days and rejected otherwise.