Use of thera and therii

Dear friends, 

What is the common usage of 'thera' and 'therii' in the context of Buddhist ordained persons?

Especially with regards to nuns ordained in the Theravada tradition observing the samanerii precepts like in Myanmar, is it prohibited for them to use 'theri' after being ordained for more than 10 years?

There are those who say that 'thera' was restricted for use of bhikkhus, and 'therii' for bhikkhunis.

Please advise. 

Thank you! 

With metta 

Dhammanandi