Both are overhead systems that use covered conductors, but they are built differently. A spacer cable system holds three covered phase conductors close together in a compact arrangement using spacers, and carries them on a separate steel messenger wire that takes the mechanical load. A covered conductor line, by contrast, uses a single-core covered conductor on poles and cross-arms in the same way as a bare line, with each phase self-supporting between poles.
APAR covered conductors are self-supporting and need no messenger wire. They can be installed on an existing bare-conductor network without changing the poles, cross-arms or insulators in the same span. A spacer cable suits very compact corridors, while covered conductor suits reconductoring existing lines. See the covered conductor page and the overhead conductor comparison.