Dept. of Reviteristics - Activate Dimensions

Temporary dimensions just show up when you select an element. As a model matures and there are more elements Revit may "decide" that displaying the temporary dimensions would take too long to calculate and instead present a button on the Options Bar called, not surprisingly, Activate Dimensions.



When you select elements, to copy or move them for example, you should also find the Activate Dimensions tool appears which will allow you to alter permanent dimensions too.

One interesting collateral benefit is you can select multiple elements that all need to move by the same relative distance by altering one dimension value on a string.