These library cells do not have signal connectivity. They connect only to the power and ground rails once power rails are created in the design. They also ensure that gaps do not occur between the well and implant layers. This prevents DRC violations by satisfying well tie-off requirements for the core rows.
Each end of the core row, left and right, can have only one end cap cell specified. However, you
can specify a list of different end caps for inserting horizontal end cap lines, which terminate the
top and bottom boundaries of objects such as macros. A core row can be fragmented (contains gaps), since rows do not intersect objects such as power domains. For this, the tool places end cap cells on both ends of the unfragmented segment.