Dear Nigel,
Regarding this parameter:
<lens type=“division” focal=“15” c1=“-0.3143”/>
It is used to set proper lens distortion priors to cameras with excessive lens distortion, such as GoPro. Other cameras with fixed lens usually don’t use fisheye lens which cause such large image distortion. So setting this parameter helps prior to camera aligment in case you have fisheye lens. Other cameras with more narrow lens such as FC350 are not affected by this so you don’t need this parameter.
In case you wish to add new camera to the sensordb there is a help section in RC about this topic or you can use this article https://support.capturingreality.com/hc/en-us/articles/115001530852-Yellow-triangle-with-an-exclamation-mark
Michal