The following article refers to functionality available in Workflow builder, which is located in:

Administration > HR > Workflows

When editing or creating a new workflow it is possible to determine whether the workflow will trigger for employees who are currently in probation or to exclude them. 

When building a workflow, you will see the following option: 

Based on the option selected this setting will determine whether or not a workflow will trigger against an employee based on what is entered for the employee's probation date. The probation date for an employee is set in:

People > Employees > Core Details > Change

Depending on the option chosen and the probationary date is entered for the employee will determine how this option works - see below for all the possible outcomes where the workflow will or will not trigger:

Please pay particular attention to how blank / no probationary date is dealt with - depending on the option chosen this can behave very differently.