Preliminary Concepts
The 'Abstain' is a configurable option of the Herzum Approval add-on.
It's possible to configure it in the Global Configuration administration screen.
Solution
The customer requirement has been satisfied through the configuration of a Abstain Approval Action from Other Customization section.
From the issue view it will appears in the Approval panel the bar status, which showed the percentage of approval actions of abstain (blue label)
and the button which it will allow to abstain
If an user abstains the panel will be showed as following.
