ONVIF
ONVIF is an open industry standard that lets IP cameras, recorders and video software from different manufacturers work together, by defining how a device describes itself, exposes its video streams and accepts commands. A camera that conforms to ONVIF Profile S can be discovered and streamed by any conformant system.
Before ONVIF, each camera brand needed its own driver in the recording software, and changing one meant changing the other. With it, a buyer can mix brands and change software without replacing cameras.
For a cloud VMS the practical part is the stream: an ONVIF camera exposes an RTSP address that the service can record from, and describes its profiles and controls in a documented way. Conformance is by profile, so check the profile the camera claims.
Camzify connects to any ONVIF or RTSP camera. The supported cameras page and the ONVIF and RTSP guide explain what to look for on a camera you already own.