General Settings

The general settings page allows you to edit basic options about a game variant, such as the score to win.

The fields available to edit are as follows:

Teams Enabled
Controls whether teams are enabled.
Player Species

Determines whether players play as Spartans or Elites. It should be noted that in team games, a player's species is what controls basically all team behavior, rather than the actual team that the player is on. The available values are:

Player Preference
Players will spawn as the biped they've selected in their customization options.
All Spartans
Player customization will be ignored, and players will always spawn as Spartans.
All Elites
Player customization will be ignored, and players will always spawn as Elites.
Use Team Species
Player customization will be ignored, and players will spawn as the species set up in their team's individual configuration.
Spartans vs Elites
The "defense" team will be Spartans, and the "offense" team will be Elites.
Round Time Limit
The maximum amount of time that a round can last. The value "no limit" exists in place of zero (i.e. to reach it, decrease the value to "1 minute" and then decrease it again).
Number of Rounds
The maximum number of rounds that a match can have. The value "no limit" exists in place of zero (i.e. to reach it, decrease the value to "1 round" and then decrease it again).
Sudden Death Time
How long Sudden Death lasts, in game variants that use it. Sudden Death is "bonus time" granted to players if someone is near or using an objective object (e.g. carrying a flag) when the round time runs out.
Grace Period

How long the Grace Period lasts, in game variants that use it. If the conditions needed to trigger Sudden Death stop before the Sudden Death time limit is reached, then the Grace Period is how long players have to reinstate those conditions before the round ends.

For example, in Capture the Flag, if an attacker is standing on top of the flag when the round time limit is reached, then they will be granted Sudden Death time. If the attacker is killed before Sudden Death runs out, then Sudden Death will stop and the Grace Period timer will begin. If the attackers reach the flag within that time, then Sudden Death will continue from where it left off, while the Grace Period resets.

Fields hidden by default

Firefight files contain the same set of Custom Game options as normal Multiplayer game variants, but not all of these options are functional. By default, ReachVariantTool will hide Firefight settings that are known to be non-functional. Those settings include:

Perfection Medal Enabled
Controls whether it is possible to earn a Perfection Medal in this game variant. Bungie and 343 Industries only enable this for pure Slayer variants; even Flag Slayer keeps it disabled.
Reset Players on New Round (unused)
A non-functional leftover. Presumably, this setting would have let you control whether players' weapons, health, or other states were reset with every new round.
Reset Map on New Round (unused)
A non-functional leftover from Halo 2. In the past, this option let you control whether a map's objects and state were reset with every new round, but testing suggests that it doesn't work in Halo: Reach.
Rounds to Win
Presumably the number of rounds that a single player or team must win to win a match early. Testing is pending.