When should the AMP$4$E TETRA engine be enabled?

Only if there are no other anti$-$virus products on the endpoint, as enabling TETRA alongside another anti$-$virus solution could cause serious degredation in the performance of the endpoint

In high$-$risk environments, on high$-$value assets, where one anti$-$virus engine might not be sufficient to catch Day$-0$ threats

Any time that additional virus protection capabilities are desired

You should not enable the AMP$4$E TETRA engine unless directed to do so by Cisco Technical Assistance Center $($TAC$).$