Generally, a Supervisor signs off evidence and an Assessor signs off the final assessment. A Supervisor can comment and either sign off and/or reject evidence.
However, in some cases the 'Supervisor' and 'Assessor' roles can overlap. Whilst the primary responsibility of an Assessor is to sign off on final assessments, if needed they can also sign off evidence. This is up to the trust/organisation.
It is the responsibility of your Unit Manager to ensure the roles of supervisor and assessors have been set within your team.
For information on how to assign a supervisor and/or an assessor for signing-off, please see the following FAQ page.