Its purely based on number of participants within the framework of the game. This game has two group sizes : the 4 man Light Party, and the 8 man Full Party.

Is it hard to understand that the definition of "raid" is "multiple groups" (or, in FF14 terms, multiple "parties") and has nothing to do with the difficulty of the encounter(s)?

Either all 8 man content is group content. Or all 8 man content is raid content. Castrum, Praetorium, Titan Extreme, Garuda Hard .. if these 8 man pieces of content are group, then other 8 man pieces of content must also be group, which includes Binding Coil. If, however, you want to call an 8 man "full party" a raid, then ALL such instances of 8 man partying are raiding, whether you are raiding level 7 FATEs in some newbie zone, or raiding Castrum, or Ultima Weapon Hard, or Binding Coil.

Since the vast majority of 8 man content is considered group, however, and not raid .. then there is only one raid in FF14 ATM, and that is Crystal Tower, with Binding Coil being group content. Which brings me back to my initial post : we need more raid (multiple Full Party) content!