The lintel joint is a location at the top of the firebox where the main chimney structure meets the brick façade front that also constructs the top of the firebox. Many times the bonding of one section of the main chimney masonry to the front wall masonry is with just normal masonry grout or grout with fireclay. Most of the time, the brick masons will not install brick ties because it is a small rise and the brick is supported by steel lintels.
The main chimney typically has its own foundation, and IRC code R1003.8 states that chimneys should not support any loads except their own weight. The front brick surround can be attached to the surrounding wall with brick ties. The chimney may settle at one rate and the rest of the home at another. This differential settlement may cause shear stresses in the bonding between the chimney and front-facing wall. Along with heat from the firebox, this non-refractory mortar heats up and deteriorates.
If this bonding joint deteriorates and forms cracks and gaps then heat can escape between the main chimney and the front wall. Many times timber stud members are installed above this location, and it can become a fire hazard, so we always recommend repairing gapped or deteriorated lintel joints with C199 refractory mortar.