From the Rulebook:
A single event typically only has one relevant Special Effect.
For instance a Destruction Arrow is both Armor Destroying and Shield Destroying, but there is no situation in which both of those matter simultaneously.
Likewise a sword may be both Armor Breaking and Wounds Kill, but players are not affected by Armor Breaking and armor is not affected by Wounds Kill.
When using a weapon or magic which causes an Special Effect you must announce it to your opponent at the beginning of an engagement and periodically during the engagement when reasonable.
Example: a Barbarian moving to engage an unarmored Scout need not announce ‘Armor Breaking’; however if an armored Paladin moves into the engagement the Barbarian should announce ‘Armor Breaking’ at their soonest reasonable opportunity.
The game flows more smoothly and everybody has more fun when Special Effects are stated ahead of time rather than kept as surprises.
See Also: Engulfing