Note 1: A PLR converts a ground, received from the E lead of one signal path, to -48 Vdc, which is applied to the M lead of the concatenated signal path.
Note 2: In many commercial carrier systems, the channel bank cards or modules have a "PLR" option that permits the direct concatenation of E&M signaling paths, without the need for separate PLR equipment.
Source: from Federal Standard 1037C