Difference between PJC:All and PJC: Expenditure Batch contexts in Project Cost Collection Flexfield

Summary:

What is the difference between PJC: All and PJC: Expenditure Batch Contexts in Project Standard Cost Collection Flexfield?

Content (required):

We are unable to understand the difference between the contexts PJC: All and PJC: Expenditure Batch in the LOV of contexts in Project Standard Cost Collection Flexfield. Also, our requirement is to open a DFF at Project Expenditure Batch Header Level, but unable to find one. So, we tried creating a UDF using PJC: Expenditure Batch context in Project Standard Cost Collection Flexfield setup, but it is not appearing on the expenditure batch header.

Any inputs would be appreciated!