A Type B 90° Elbow is a double‑wall vent fitting used in gas venting systems (also called B‑vent, B Gas Vent) to change the direction of exhaust flue gases by 90 degrees (a right angle). It is used where vent routing requires a turn—e.g. to avoid obstructions, align with wall penetrations, or change from vertical to horizontal or vice versa.

Because it's a “Type B” component, it is meant for use with gas-fired appliances (e.g. gas furnaces, boilers, water heaters) under certain temperature and pressure conditions (Category I type appliances). It’s not for use with wood or high‑temperature exhausts (unless explicitly rated).

The elbow is prefabricated (rigid) or adjustable (telescoping / articulated) so you can set the angle precisely. It maintains the proper clearances to combustibles and ensures safe venting of combustion gases through the proper inner liner, while the outer wall protects the structure from heat.

For your “3.5‑inch diameter” case, that might refer to the outside outer shell (outer wall) dimension, while the inner flue might be nominal 3" (common in B‑vent systems). For example, I found a 3” Type B 90° elbow listing which states an outside diameter of ~3.5".