MGs are assigned to ‘tiers' based on their level. Levels 1-99 are tier 1, 100-199 tier 2, 200-299 tier 3, 300-399 tier 4, 400-499 tier 5, 500-599 tier 6. (No MGs are currently over 599, although there is no hard limit.)
Vaults, Grimoires, Items, and Spells also have tiers. An MG can only buy things of their tier or lower. For example, Naiad, at tier 2, just under 200, can only by tier 1 and 2 spells, not tier 3. Mai, and all other MGs less than level 100 can only but tier 1 (Common) items. This would seem to be the main reason that multiples of 100 are major milestone levels.
For “nonesoteric” items, the tiers are named: Common, Rare, Masterwork, Arcane, Legendary, Mythic. Selene ch.5