I'm making a Tibbit character, who is a monstrous humanoid. So I went to look up the creature type on d20srd. A humanoid is listed as having an 8-sided hit dice or by class, meaning that class overrides the creature type's hit dice. Okay, but Monstrous Humanoid type simply says they have an 8-sided hit dice. Does that mean the racial type's hit die takes precedence over the class?
I think you're conflating two different things. Think of monstrous hit dice like class levels in the class "monstrous creature". So, if you have e.g. four hit dice as a monstrous humanoid and three as a wizard, you have 4d8 + 3d4 hit points (and probably also have an ineffective character).
There is a specific exception to this where humanoids with exactly one hit die instead replace that hit die with their first class level.
Tibbits, along with most player races, don’t actually have Racial Hit Dice, but most creatures (particularly unplayable monsters) have them, in some cases exclusively.
Compare the elf monster entry to the troll monster entry: the elf warrior lists only the 1d8 hit die of the warrior class, but the troll hunter lists both the 6d8 of “being a troll” as well as the 6d8 of “being a 6th-level ranger.” In addition, notice this at the bottom of the troll page:
Trolls As Characters
Troll characters possess the following racial traits.
- Racial Hit Dice: A troll begins with six levels of giant, which provide 6d8 Hit Dice, a base attack bonus of +4, and base saving throw bonuses of Fort +5, Ref +2, and Will +2.
Note that the basic troll is not a “troll warrior,” but just a troll. The basic troll has no class levels; it has only racial hit dice. A troll has six hit dice just because it’s a troll, even with no class levels. If that troll hunter had taken levels in barbarian instead of ranger, it would have 6d8+6d12 hit dice; if it had taken levels in wizard (as poor a choice as that would be), it would have 6d8+6d4.
A creature with no racial hit dice, like elf or tibbit, must have at least one level in at least one class, because no creature can have zero hit dice. With no racial hit dice, they must have class levels instead. But they use their class hit dice (and other features), not the statistics of the racial hit dice associated with their type.
In the end, the Monstrous Humanoid type of the tibbit provides the Traits:
A monstrous humanoid possesses the following traits (unless otherwise noted in a creature’s entry).
- Darkvision out to 60 feet.
- Proficient with all simple weapons and any weapons mentioned in its entry.
- Proficient with whatever type of armor (light, medium, or heavy) it is described as wearing, as well as all lighter types. Monstrous humanoids not indicated as wearing armor are not proficient with armor. Monstrous humanoids are proficient with shields if they are proficient with any form of armor.
- Monstrous humanoids eat, sleep, and breathe.
But not the Features, which are features of each racial hit dice:
A monstrous humanoid has the following features.
- 8-sided Hit Dice.
- Base attack bonus equal to total Hit Dice (as fighter).
- Good Reflex and Will saves.
- Skill points equal to (2 + Int modifier, minimum 1) per Hit Die, with quadruple skill points for the first Hit Die.
Those Features are used only by monstrous humanoids that have racial hit dice, for example hags.
No, racial hit dice are only used when either playing as a monstrous race or advancing a monster by Hit Dice.
Playing as a monstrous race means choosing a race that has more than one racial Hit Die (for example, a monstrous humanoid with 2d8 base HD). When doing so, you do start with all of that race's racial Hit Dice, but this doesn't apply to your situation. A Tibbit has only a single racial Hit Die, like Humans or Halflings, and so instead replaces their starting Hit Die with their class Hit Die. (This is a good thing - it means you don't waste an entire level on "being a Tibbit" before you start getting your class abilities.)
Player characters almost never advance by Hit Dice, gaining class levels instead, but a DM can make a monster stronger by giving it more Hit Dice, in which case they use the monster type's HD size.
TL;DR: Players almost never use the HD-by-monster-type, using class HD instead. That's more of a DM thing.