You've described the Guard as the Federation's equivalent to the National Guard. Based on that, I'm going to assume that the Guard's purpose is to act as a militia of sorts. They focus on patrolling and protecting Federation territory and worlds, and their purpose is to lift some of the pressure off of Starfleet, so that the latter can focus more on external threats and exploration. When Starfleet goes on the offensive, and takes the fight to the enemy, the Guard is charged with protecting Federation space, and attacking any enemy fleets that are sent into Federation territory, assuming that they aren't intercepting Starfleet armadas. The Guard's budget isn't quite as large as Starfleet's, and their ships aren't as powerful as their Starfleet analog equivalents. For example, the Sampson class wouldn't be able to handle as much as a Starfleet Detroyat-class Heavy Destroyer, with the latter being far larger, and a little more heavily armed. So anything that the Guard can't handle, Starfleet comes in and mops up, but until then, the Guard holds the fort as best they can. Is this about right?