Contract Billing Questions

Hello,

1.When I set up a contact for T&M entry I specify cross reference table and accounts  that contain related cost. For example 9000 account

2. next, I create a Mark Up table and specify accounts to that are used to create a mark up . These are different form cross-reference.for Example 9950 account

3. When I run GL extract .

My understanding that Mark-up account is used to read F0911 table and create mark-up records for Billing.

What is difference between a cross-reference table from T&M Billing lines and Mark-UP table? How do they used?