What is the difference between a planetoid and an asteroid?
- Need to add a section to define this matter.
Technically, best as I can tell, different names for the same object. Semantics I guess. Both are also referred to as "minor planets".
- In Traveller terms, planetoid seems to be the preferred nomenclature. Planetoid belt may refer to an inhabited belt, while asteroid belt seems to refer to an uninhabited one, but I'd really like to find the cite for this ... that I may possibly have imagined ...
- Any minor planet has a size code of S (less than 800 km diameter). Might be worth thinking about the lower range of this (meters?), perhaps also tying in references to planetoid hulls.
- Density of the object would be something worth discussing too. Current thinking seems to suggest that most are little more than associations of dusty rubble, which would be just useless for making hulls out of.
Found some useful cites:
Asteroid and Planetoid Belts: The terms asteroid and planetoid are effectively synonymous; they mean small or minor planets. Each term refers to a belt of many similar small planets in orbit around the central star.
- For the purposes of differentiation, the term asteroid refers to such a belt when it is the main world of a system. The term planetoid belt refers to such a group of minor planets when another belt or world is the main world in a system.
- Asteroid and planetoid belts hold between 1,000 and 10,000 asteroids or planetoids each. 
CT Book 6 Scouts page 39 (earliest canon reference)
Belt. An asteroid belt (a mainworld) or a planetoid belt (other than a mainworld).
T5 v5.10 Core Book 3 page 16 (most recent canon reference)
Thank you, sir.