A .ctt is a control template. Use it as a template to create other controls (.ctls) based on it. If a control is set to be a typedef that's not the same as it being a template (has some similarities though)...
In everyday use it's the .ctl that you should use and edit. Just change the ending of the file name if you don't want it to be a template/want it to be a template. Creating the files is done from the customize control interface. When editing a control (select it and choose customize control from the edit menu) you can save the control either as a control or a template.
