Initiative phases for mechs and vehicles are based on their MaxWalkDistance variable found in their movement file.

For mechs:
Phase 5:  209.0, aka the special phase.  Locust, Spider speed or faster
Phase 4: 164.0, aka the light mech phase. Phoenix Hawk, Vulcan speed or faster
Phase 3: 139.0, aka the medium mech phase.  Dragons, Quickdraws, Griffins, Shadow Hawks and so on.
Phase 2: 119.0 aka the heavy mech phase. Battlemaster, Blackjack, Centurion, etc.
Phase 1: anything slower than the above.  Atlas, King Crab, etc.

For vehicles:
Phase 5: 229.0, or anything even faster than Locust.
Phase 4: 189.0 or Jenner speed and up.
Phase 3: 164.0, or Phoenix Hawk speed and faster.
Phase 2: 139.0, or Griffin speed and faster
Phase 1: all vehicles slower than Griffin.

The cut off points can be adjusted in the settings.json file to suit your own preferences.

Works with equipment like the Cyclop's lance initiative buff and pilot initiative buffs.  However, initiative is limited to just those 5 phases, so a Spider Pilot with Master Tactician in a lance with the command Cyclops will still be moving in the first phase.