The Enable Abstain Approval action option under the Other customizations section of the Approval Global Configuration screen allows to have the additional option ABSTAIN beyond Approve and Reject.
By default the allowed Approval actions include the APPROVE, REJECT and RESET actions.
The following screenshots show, on the right, the issue screen and, on the left, the associated setting to enable the Abstain option.
Approval Global Configuration screen
Approvals section on the Issue screen