3DS Max Snapping

2D Snapping - snapping on same plane
Z axis

2.5  D Snapping - Sanpping objects in space as if they are on the same level
Top Viewport; not perspective viewport
this Top Viewport works because of the axis these objects are in.\

3D Snapping - Move object in space to exact x, y, and z coordinate
Its a generalized snapping method. That isn't as controlled as 2D and 2.5 D

Rotational Snap Toggle



 2D Snap
The cursor snaps only to the active construction grid, including any geometry on the plane of that grid. The Z axis, or vertical dimension, is ignored.

 2.5D Snap
The cursor snaps only to the vertices or edges of the projection of an object onto the active grid.

Suppose you create a grid object and make it active. You then position the grid object so you can see through the grid to a cube further off in 3D space. Now with 2.5D set, you can snap a line from vertex to vertex on the distant cube, but the line is drawn on the active grid. The effect is like holding up a sheet of glass and drawing the outline of a distant object on it.

3D Snap

This is the default tool. The cursor snaps directly to any geometry in 3D space. 3D snapping lets you create and move geometry in all dimensions, ignoring the construction plane.

Source:
https://knowledge.autodesk.com/support/3ds-max/learn-explore/caas/CloudHelp/cloudhelp/2019/ENU/3DSMax-Basics/files/GUID-66F42B65-7EA5-4CF8-BBF1-46EBC0845C75-