There's been a kerfuffle lately about how the Multi-Tracker works on Tau Empire Battlesuits, which has gotten some people very confused about Monstrous Creatures, possibly because there are Monstrous Creatures that can take Multi-Trackers.
"Basic rules apply to all the models in the game,
unless specifically stated otherwise. They include the rules for
movement, shooting and close combat as well as the rules for morale,
found between pages 10 and 31. These are all the rules you'll need for
your average Infantry model.
"The advanced rules that apply to a unit are indicated in the entry for the unit in their relevant Warhammer 40,000 codex.
"Where advanced rules apply to a specific model, they always override any contradicting basic rules." (General Principles, Basic Versus Advanced, 6th Edition Rulebook, p.7)
otherwise stated, if a model has more than one Shooting weapon, he must
choose which one to shoot - he cannot fire both in the same Shooting
phase." (More Than One Weapon, 6th Edition Rulebook, p.51)
"Monstrous Creatures can fire up to two of their weapons each Shooting phase..." (Shooting, Monstrous Creatures, 6th Edition Rulebook, p.48).
Overwatch attack is resolved like a normal shooting attack (albeit one
resolved in the enemy's Assault phase) and uses all the normal rules for
range, line of sight, cover saves and so on." (Resolve Overwatch, 6th Edition Rulebook, p.21)
More Than One Weapon restricts Infantry, the basic case, to firing up to one weapon per Shooting phase, and hence up to one weapon per Assault phase. Monstrous Creatures override this basic rule, but restricts Monstrous Creatures to firing up to two weapons per Shooting phase, and hence up to two weapons per Assault phase. Therefore, Monstrous Creatures can fire up to two weapons during Overwatch.
"A model with a multi-tracker can fire an additional weapon in each Shooting phase." (Support Systems, Codex: Tau Empire, p.69)
More Than One Weapon again restricts Infantry, the basic case, to firing up to one weapon per Shooting phase, and hence up to one weapon per Assault phase. Multi-Tracker does not override this basic rule, but allows bearers, Infantry and Monstrous Creatures alike, to fire an additional weapon in each Shooting phase. Therefore, Infantry with Multi-Trackers can fire two weapons during the Shooting phase, and one weapon during Overwatch.
So what's the difference, precisely? Well, take the basic rules, covering Infantry. An Infantry model can fire up to one weapon per Shooting phase; this is the normal shooting attack. Let us call it 's.' A Monstrous Creature model can fire up to two weapons per Shooting phase. This is twice the normal shooting attacks. It is '2s.' A Multi-Tracker allows an additional weapon in each Shooting Phase. This is not the normal shooting attack; it is an addition to the normal shooting attack(s). This is a bonus, a +1 limited to the Shooting phase. It's the fact that it's an additional weapon, an addition to the normal shooting attack, rather than an increase in the normal shooting attacks, that means it is restricted to a bonus in the Shooting phase rather than in both the Shooting phase and the Assault phase.
An Infantry model with a Multi-Tracker fire s+1 in the Shooting phase and s in the Assault phase. A Monstrous Creature fires 2s in the Shooting phase and 2s in the Assault phase. A Monstrous Creature with a Multi-Tracker fires 2s+1 in the Shooting phase, and 2s in the Assault phase.