This page outlines most tags given by the microprofiler really well. It is missing some tags some developers could really use help with, especially when making large maps with shadows, mesh parts and various textures.
Most notably, the following:
Shadow
depthPass
ID_Transparency
ID_Opaque
ID_Decal
There are likely others I’m missing, but these are all most notably missing the most documentation and methods to improve their performance.
Shadows: rendering shadows; to optimize change Lighting.Technology or set CastShadow of small parts and high-poly meshes to false
Depth pass: rendering the scene, related to shadows; optimize similarly to shadows
Id_Transparent: rendering transparent parts; to optimize avoid multiple layers of transparent parts/particles and change the transparency to 0 where possible
Id_Opaque: rendering opaque parts (transparency 0); to optimize avoid high-poly meshes (you can use Wireframe rendering mode to find expensive meshes), and remove small parts.
Id_Decal: all the decals; to optimize limit the amount of decals where possible
More interesting are lighting tags, and even more interesting are physical tags which are not documentated anywhere.
It’s also hard to find it out as we don’t know how Roblox handles the physics internally and the tag names seem vague.
In the mean time for anybody finding this thread because they searched for any of the terms I included, this developer sourced documentation on the microprofiler outlines them well.