I have been looking for a suitable transformer for a project I am working on.

Is there a difference when the primary and secondary impedance are listed with a "CT" after the number, as in 600:600 and 600 CT: 600 CT?

 

CT is Center-tap. It is a winding with a center-tap that you don't need to use.

 

While you may not need to use the center tap, it could open up other possibilities.
A 600 CT: 600 CT could also be used as:
300: 600 CT
600 CT: 300
So, you could double or half the output voltage, or simply match the impedance differently. It all depends upon what you need to do.

 

Remember also that the 600 ohms or whatever is rated at the frequency of operation.

You will not measure this with a DC ohmeter.

Ohms law (AC version) then gives the normal current in any configuration you use.