EXFO FTB-800 Series for FTB-1 - Manual (Page 304)

Test Results
294 FTB-700G/800 Series
Alarms/Errors

REI-L (Remote Error Indicator - Line)
RX: Bits 5 through 8 of the M0 byte contain one pattern from the
following binary range: “0001" through “1000" (1 to 8) (located in the
first STS-1 of an STS-n signal).

REI-P (Remote Error Indicator - Path)
RX: Bits 1 through 4 of the G1 byte contain one pattern from the
following binary range: “0001" through “1000" (1 to 8) (located in every
STS-1 of an STS-n signal).