How to tag One Custom filed for One Enterprise project type in project Online?

Thank Paul_Mather for your reply,

Whatever you said is correct. We have to use Departments concept in POL side. When comes to MS Project it will show all the fields. We don't have any other alternative now.

Here I am facing few issues in departments concept Paul. 

for EX:  I created 2 custom fields in  Project Level Entity, department values are  Hardware and software. I assigned department for  Custom filed A as hardware and Custom filed B as software. I assigned the department as Software to a project.

If I open the project in MS Project, it will shows 2 custom fields with data. But it is uneditable why because  we tagged it as Project entity.

Same concept , When i use 2 custom fields in a task level entity & assigning the department for  Custom filed A as hardware and Custom filed B as software. I assigned the department as Software to a project.

If I open the project in MS Project it will show both fields ,but two fields are editing ,I don't know why. Even though I mentioned Software as a department for that project. It has to edit only Software fields not Hardware fields. Here Hardware custom filed also editing.