The Sekirei Crest is proof of a Sekirei commitment and loyalty to their Ashikabi. The Sekirei Crest appears on the back of the neck of any Sekirei who has already kissed their Ashikabi. If a Sekirei's Crest disappears, it means that the Sekirei has been defeated and they are no longer allowed to participate in the Sekirei Plan and are not allowed to be with their Ashikabi anymore. Though, there are exceptions like Akitsu, a disused product, who has stated that her master, cannot place his mark on her, meaning he cannot actually be her Ashikabi. She also tells Homura that she is envious of him since he can still be marked by an Ashikabi.
Making the crest disappear
There are two ways to make the crest disappear which will defeat the Sekirei. The first is to cause a lot of damage to the Sekirei. After a certain amount of damage is taken, the crest will disappear and the Sekirei becomes immobile. The actual amount of damage needed may vary between Sekirei, as some in the series have been defeated by a single punch, where others like Musubi have taken a ton of damage before having their crests disappear. The amount of damage a Sekirei received is categorized in 5 so called shutdown levels. The higher the level the more severe was the damage taken which caused the termination of the Sekirei. The other way to make a crest disappear is to touch the mark and to recite a prayer unique to each Sekirei. The only example of these methods not working was when Musubi's crest disappeared and she became immobile. It was then that No. 08, Yume came out and saved Musubi, also restoring her Sekirei Crest in the process.