Every arrangement that touches bitcoin can be drawn as a diagram with a line through it. On one side, the actions the holder can take alone: spend, move, sign. On the other, the actions that need someone else: release from a peg, withdrawal from an exchange, a bank honouring a redemption.
The Gazette drew the boundary for Liquid in its Story and used the same diagram in The Satoshi Brief No. 002. It is a habit rather than a definition: before a claim about "your keys" is accepted, the boundary is drawn and the exit is located.
Where the exit sits on the other side of the line, the balance is a claim.