How to tag vertices/triangles as "growable"?

I'm trying to grow some hair procedurally on a 3d mesh and I thought the best way would be to somehow tag the vertices or triangles inside 3dsmax as "growable" but I'm not sure how to do that.

There's material groups and I got them to work so I have two materials for the mesh in Unity but I don't understand how do I differentiate what vertices belong to what material in code.

You can use vertex colors as 'tags' - simply paint the desired vertices with a specific color in your 3d app. You can then access that color information either in the mesh interface or in a custom shader to make those vertices grow.

For meshes which use more than one material in Unity, the mesh is divided into "submeshes". Each submesh uses one material from the array of materials stored in the object's renderer.materials array.

You can use the Mesh.GetTriangles function to get information about which triangles belong to which submesh.

duck is right about submeshes. to grow hair or ... you can not tag triangles in unity but you can use the submesh as duck said if all of your triangles are in a submesh. also you can use mesh morphing. define different hair meshes and morph the triangles of your submesh to the new hair size when you want to grow it.