The detail of External PCIe cabling specification?
From this user guide for an external PCIe product, it describes the signals as follows:


PWR: Power to support AOC and signal conditioning components within the cable assembly.
MGTPWR: Power supplied to the connector for cable management components that are needed while the link is not active. This needs to be active if the subsystem has power.
CBLPRSNT#: Cable present detect, an active-low signal pulled-down by the cable when it is inserted into the connector.
CADDR: Signal used to configure the upstream cable management device address.
CINT#: Signal asserted by the cable assembly to indicate a need for service via the CMI controller.
CMISDA: Management interface data line. Used for both initial link setup and sideband messages when used with CMI compliant cables.
CMISCL: Management interface clock line. Used for both initial link setup and sideband messages when used with CMI compliant cables.