hi...
In SOC Encounter end cap definition is
End-cap cells are preplaced physical-only cells that are required to meet certain design rules.
They are placed at the ends of the site rows, and are used in some technologies for power.distribution. End-cap cells are placed in a preplaced status, so future placement commands do not move them. Add end-cap cells to the design before any other standard cells are
placed, but after hard blocks are placed in the floorplan.
In addition to that
Endcap cells are placed at the end in order to avoid routing of macros outside the core.
if u want in detail refer this