Test reports have for a long time had a Fixture id field, which is used to log which test fixture was used to perform the test. This can be relevant as fixtures can be faulty, and logging it with the test enables you to spot that a fixture is a common denominator in a set of failed tests.
Fixture id is being superseded by assets, but will continue to exist for the foreseeable future.
Assets is a newer feature. It's purpose is to track usage and maintenance of equipment used in test, including fixtures. Starting with WATS 2022.1 and WATS Client 6.1 you can log which assets are used in a test. The test report will show information about the asset based on how the asset is configured in Asset Manager and what the asset's state was when the test was performed, i.e. usage count, how long since calibration and until next calibration, and how long since maintenance and until next maintenance.
This information gives more insight into how a fixture or other assets affect test results than the fixture id field can. An additional advantage of logging a fixture as an asset is that a test report can include multiple assets, but only one fixture id.
New versions of WATS will include asset in more analysis, for example as a dimension in Dynamic Yield and in Step Details.