A relay controller have one or more on/off capable channels. Different HDL Models have different number of channels, and you see this in hdl_devicelist.js under each type as "channels". You always add each separate channel as a Homey device, and they will as such be shown indivdually in the process of adding them.
ON/OFF switches are a built-in capability in Homey, and as such have very good Homey support.