"t" is the variable that drives values from the From-To settings.
There is some relevance between T and the accuracy settings of your current part or assembly. not one to worry about unless you are looking for a pure function. For instance, an involute will have a varying start angle depending on your accuracy settings.
"t" is similar to trajpar in variable sweep sections. Trajpar is limited to values between zero and one; equations can use any value including negative. Beware however, that sometimes these values are not followed completely if the equation has a problem. Check you work... and the solution Creo is presenting.