Use the Custom (Calc) filter when a custom evaluation is needed.
Use PHP notation to create a calculation.
For example: If you Trigger on New Comment for an item in a Help Desk App, you may wish to add a filter for the flow to be triggered ONLY when the "Help Desk" App makes a comment.
Start the Trigger on New Comment and add the Custom (Calc) filter.
Create a PHP expression for your conditions. Use the Token Selector to select the required fields.
Complete flow by adding Actions to complete your workflow steps.
This flow will trigger when the "Triggering Comment Creator" is equal to "Help Desk".
Check out the available PHP functions