When a billing or revenue limit is displayed in italics it means that the limit is shared.  There is at least one other project that can spend against the limit.

Italics billing limit example

What is the difference between billing limit and budget?

The billing (or revenue) limit is an expectation that comes from the project's sponsor whereas the budget is an expectation that is established by CU.  Often these are the same, but there are cases where they are not.