Test to Release applies to anyone travelling back from a red or rest of the world country who are NOT vaccinated.
The test to release is a government scheme to shorten your quarantine period. This is a PCR test to be performed on the fifth day of your arrival in the UK.
You do not need to do a test to release if you are vaccinated as you only need a day 2.
So, if you are returning from holiday and are not vaccinated but don’t want to be quarantined for a full 10 days, perhaps you need to return to work, you can take a test to release on day 5.
You still need to take your day 8 test, but you will be able to end your quarantine early upon a negative test result.