I've found a clear explanation of this type of fitting....

Fitting Types:
Normally, fittings fall into three basic types or categories. These are In-line, On-line and Closures. The In-line fittings include elbows (Ells), Tees, Couplings and Reducers. The On-line fittings include a wide variety of "O-Let" fittings used primarily for making branch connections. ....

O-let fittings:
Another way to make branch connections on pipe and vessels is by using an "O-Let" fitting. An "O-Let fitting is designed for use on 3" and larger welded pipe. The main feature of the typical O-Let fitting is the built-up base design which eliminates the need of any other form of branch reinforcement. The O-let fitting is manufactured in a number of styles.