What is diffrence between project cost plan, project revenue plan etc

Hello Gurus

I want to know what is project cost plan, project revenue plan and total cost commiment.

and also want to know what is diffrenece between them,

because some time there is budget amout is remaining but system does not provide me to make PO against that project.

s i would like to know what is the actual problem.

Please reply me.

Thanks & Regard,

Riten Patel