Some publishing companies demand that all authors sign a release form prior to submitting their work.
This is not a standard procedure in publishing.
It’s done to protect the publishers against plagiarism charges should they go on to reject a book and subsequently publish something similar. It seems to have crept in from the screenwriting world where release forms are more more common: but with the copyright laws as they stand, publishers don’t really need this extra level of protection.
It is, however, a useful red flag for writers. If a publisher asks you to send a release form with your submission you would be wise to reconsider submitting to them. Because it’s not a standard procedure, it’s a good sign that the publisher might also follow dubious practices elsewhere; and while most of the releases I’ve seen are perfectly benign, some grab all rights to the work, in perpetuity, no matter where it is eventually sold, or who makes the sale.