relative: using x, y offsets from lower left of floorplan(s)
whether to use absolute or relative position really depends on your application
assuming you have the required minimum firmware for V3, perhaps you need to have early beta access? simplest way to check is probably to just open a case and ask if that is why you see no location data for V3
You should know which version of the API output is being generated - It's a while since I set mine up but, when you create a post URL (destination for the output) the API version is selectable, via a dropdown.
If you see only v1 or v2, I suspect you don't meet the pre-requisites for the Beta. Beyond just having MR Access Points, this is down to firmware:
FirmwareR26for WiFi observations
FirmwareR27.5for BLE observations
One of the major advances with v3 is that it consolidates observations to a single POST (and location) per network, rather then getting different values, at slightly different times, from different APs.