For anyone confused about this, be aware that there are several of those Enums that are exclusive to the terrain (can’t be applied to BaseParts). RBLX Devs are actively working on an overhaul of Roblox materials that should give us more material options and more options that can be used with either terrain or BaseParts; however, it doesn’t look like they’ll remove that barrier completely (so expect to continue seeing materials in the full Enum list that can’t be applied in all the ways you would like).
Recent(ish) announcements:
New Terrain, and Parts, and Built-In Materials, Oh my! - Updates / Announcements - DevForum | Roblox
New Terrain and Parts Materials Preview Build! Oh YES! - Updates / Announcements - DevForum | Roblox
New Terrain and Parts Built-In Materials - Release Build - Updates / Announcements - DevForum | Roblox