An adoption is when you play another person's character and intend to keep this character permanently. The original creator of the character will relinquish all control and you will be responsible for the character's sheet, design and role play posts.
Some characters offered for adoption may already have a sheet. This may have been made by the original creator (if the character isn't played) or the previous driver (if the character existed before and is changing hands for one reason or another). As a general rule you are not required to rewrite the sheet. However, it needs to be resubmitted with a new role play response written by you.
