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As I know, any valid logged in user can go to approval central and perform approve/reject operation provided:
1. The user is the approver of a ticket
2. The user is an alternate approver for the approver of a ticket
3. The user has override admin or full admin access
On approval central, when a signature is seen, there is a hyper link to view more details of the ticket. If this link takes the user to the main ticket form (configurable in ap:form record) in that case appropriate apps / request / ticket level permission will be needed. But such will not be needed to approve/reject from approval central table itself.