If otherwise unspecified, does a unit with a reaction need to participate in an engagement in order for its reaction effect to occur?

For example, Imperial Officer [0074] increases the Death Star dial by 1 in reaction to a destroyed enemy objective. Does this unit need to be participating in the engagement during which the objective is destroyed in order to gain the opportunitiy to spend the resources on this benefit? Or can this benefit be activated any time after this card is in play?

Similarly, Grand Moff Tarkin [0063] has the ability to reduce the required damage to destroy an objective by 1. Does he need to participate in the engagement to gain this benefit? Or is it always in effect once he is in play?

Thanks in advance.

No. If a reaction requires for it's unit to be participating, it'll say so. Grand Moff Tarkin's effect is on as soon as he is in play (and playing him can destroy objective that had 1 remaining damage capacity).

Excellent. Thank you!