The available description of the diplomatic status Peace Pact [with my guesses added in brackets] is
Available [among Pact member heroes (unless both heroes belong to the same player)]: Spying, reinforcing. [Does not allow sharing of troops between the two alliances.]
Disabled [among Pact member heroes]: Plundering, onslaught
Alliances cannot declare War on each other within 7 days after breaking the Peace Pact.
[Member heroes can freely attack or reinforce all other domains without regard to their diplomatic status with the other alliance in the Pact.]
The available description of the diplomatic status Union [again w/ my guesses] is
Disables any kind of aggression[--i.e., spying, plundering, onslaught--among Union member heroes].
[Permits reinforcing among Union member heroes (unless both heroes belong to the same player).]
[Does not allow sharing of troops between the two alliances.]
Union alliance [or rather, a Union member hero] can attack enemies of his allies [i.e., those at War with the other alliance in the Union],
but enemy can't attack them [i.e., such a member hero] unless he [said enemy] declares War [on such a hero's own alliance, or his alliance declares War on such an enemy].
[As with Peace Pacts and Truces, alliances cannot declare War on each other within 7 days after breaking the Union.]
This compares to Alliance:
[Disables any kind of aggression--i.e., spying, plundering, onslaught--among Alliance member heroes.]
[Permits reinforcing among Alliance member heroes (unless both heroes belong to the same player) and sharing of troops with Alliance Commanders.]
[Places no restriction on aggression among former members and current members.]
Are my guesses correct? If not, please supply corrections.