The idea behind using expressions is to make sure your final units are consistent with the physical quantity you are trying to write an expression for. In fact, expressions are validated to ensure they have consistent unit types
Here is the link to the Fluent User Guide, where you should find all the necessary information. https://ansyshelp.ansys.com/?ReturnUrl=%2FViews%2FSecured%2Fcorp%2Fv202%2Fen%2Fflu_ug%2Fflu_ug_expression_syntax.html
Regarding your question related to a square pulse, perhaps, you can use IF conditions for the pulse. Please let us know if this works.