Cascade
Cascades are used when several fireplaces in a boiler room are connected to each other and the exhaust gases of all connections are led to the outside with an exhaust system.
As a rule, cascades are operated sequentially or are coordinated with one another. Merging multiple modules allows better performance, as each connected boiler performs separate tasks. For example, if one device fails, another can provide the power and the heating mode does not have to be set. Cascade controls are often installed in multi-family homes or large commercial properties because of their efficient heating characteristics
More information can be found here: