Why when I use my watch for an activity it records a training load. When I use a 3rd party app such as Zwift for cycling, or ErgData for rowing, it does not create a training load. All the same data is there, but one does and other doesn’t. Why ?

Only compatible 3rd party apps deliver data that update the Training Status, and Zwift is among them (ErgData is currently not listed at Why Is My Training Status Not Updating on My Garmin Device? | Garmin Customer Support though). You still need to use a powermeter and a HRM at cycling activities in order to get a Training Status update. More details in the mentioned document.

So I use my EPIX2 for HR and the power meter is in the ErgData from the PM5. Therefore all the data that the watch collects, is the same as ErgData collects, so therefore why does ErgData not produce a  training load ?  ErgData is actually better, as it contains more accurate and additional data. So perhaps you could put this on an update list to have ErgData providing a training load. 

Try asking ErgData to speak to Garmin, and make their app data compatible with Garmin's requirements, and the app certified. Here you are on the user forum, and we cannot help you with that.

Also keep in mind that it's only relatively recently that training status has been available outside of a Garmin device. It wasn't until summer 2021 that Garmin began to support *some* metrics being calculated for activities from *some* 3rd-party apps, and it required that the activity be synced to a Garmin device, where the data was processed to calculate the metrics, and then a second sync back to Garmin Connect. And it was only this spring 2023 that these metrics for 3rd-party app activities started being calculated directly on Garmin's servers without requiring the data be synced thru a Garmin device (though the user must still have an eligible Garmin device in their account).

So all that to say that getting any training status metrics at all from 3rd-party apps is both relatively recent and still evolving. The list of Garmin-authorized apps remains short: Zwift, Rouvy, TrainerRoad, Wahoo Systm (/Sufferfest), and of course Tacx (Garmin-owned). (RGT was not authorized however some users found a workaround, however that's moot now since Wahoo is shutting down RGT.)

More discussion in this thread about Garmin authorizing 3rd-party apps:

forums.garmin.com/.../1666169

Has this now been fixed as I noted the other day that my concept2 data now produces a Training Load. Can anyone at Garmin confirm that this bug feature has been added.