Surface

Surface is the side of a brick. There are 9 kinds of surfaces. Certain surfaces have different properties.

Stud
Studs are akin to the connectors on LEGO bricks. They appear to be bumpy, but are 2D.

Inlet
Inlets are akin to the bottom of a LEGO brick. Inlets will only connect to stud, weld, and universal surfaces.

Smooth
Smooth is a plain surface type with no texture. It's subtype, Smooth Without Outlines, is almost identical, the only difference being that it removes the outlines of (if they have any at all).

Weld
Welds have X's on them, and can hold onto any block it's touching without being anchored.

Glue
Glue is a deprecated surface type that resembled clear, squirted on glue. It originally could only hold on to Sphere Parts. If somehow selected in the Studio, it turns into a Weld.

Universal
Universal is a combination of stud and inlet. It can connect to inlet and stud.

Hinge
Hinge is a non-automatic motor. It will only spin by gravity.

Motor
Motor is automatic, if put on Constant. If you connect another brick, it will spin if unanchored.

Deprecation
In the most modern games, you don't see surfaces too often anymore, perhaps it must be because there is already the "join surfaces" tool, which is much easier to apply instead of changing each surface of each part, on the hinges and engines, another HingeConstraint class already exists, which works better.