When you enter a length value Revit will accept the value regardless of the project units your project is using as long as you provide a unit of measurement symbol.
For example, if you are working in a metric project and the base linear unit of measure is millimeters you can type 2500 and Revit will assume that you intended it to mean 2500 millimeters. If you type 2.5m however Revit will forget the millimeter assumption and use Meter as your unit.
If you are in an imperial unit project you can still enter a metric value by including mm, cm, m etc. Same is true in the reverse...in a metric project enter 12' 6" and you'll get the soft metric (mathematical) equivalent of 12 feet and six inches.
This "thinking" can usually be applied to other units of measure as well, in different parameters.