Base lining: Process by which the quality and cost effectiveness of a service is assessed, usually in advance of a change to the service. Base lining usually includes comparison of the service before and after the Change or analysis of trend information. The term Benchmarking is normally used if the comparison is made against other enterprises.
If the company has different projects. For each project there will be separate test plans. This test plans should be accepted by peers in the organization after modifications. That modified test plans are the baseline for the testers to use in different projects. Any further modifications are done in the test plan. Present modified becomes the baseline. Because this test plan becomes the basis for running the testing project.