Fenix 6 trail running vs running

VO2Max is the only difference as far as I know, and the only reason to switch between the two.

I use Trail Running any time I go for a run that I don't want to impact my VO2, for example when I'm doing a type of running I don't normally do (like full-on short interval sprints, or going for a light jog with a slower runner, or.... trail running).

As far as I know, there's no difference with GPS signal or anything like that.

C.sco is correct AFAIK; it's also a convenience for separating road from trail in your logs, and having different sets of data fields for the two (ascent and descent fields like vertical speed figure a lot more in my trail settings than road!)

Where I live it's quite windy. When the wind is up I use it since I need to work much harder into the wind.

So really... IF you actually do a trail run, and want it to count to Vo2 - you should use running? lol... 

I have always just used running whether I trail or road run as all exercise counts towards VO2max. My VO2max has hovered around mid- to high-40s for years.

The perceived need to change is because people focus on VO2max as an acute metric when in reality it is a chronic metric that needs to be reviewed over several weeks to identify trends following a specific block of training.

The VO2Max calculation works by comparing your pace to your HR (as % of maxHR).

On an uneven trail surface, your body is working harder to stay stabilized and balanced, which breaks down the relationship between pace and HR, and thus any VO2Max calculation would be misleading.

Trail Runs still count towards Training Load, but if you want a VO2Max estimate, yes, you should use the Running activity - just pick a smooth surface. VO2Max should change only gradually - once every couple of weeks should be enough to track progress.

I do this.  It feels like cheating but certain types of runs, in my case trail runs, are a lot harder and the VO2 calculation doesn't seem very smart.  Basically, it sucks to go out, do a good trail run and have your score go down.  When, instead, you can go out do some intervals and magically be 3-years younger.

I like being 20+ years younger than my actual age according to Garmin.  I'm certain it's the one thing they get right.

Not cheating in my book since you are recording a trail run as a trail run. I live in the mountains and mostly do trail running and wonder whether the MTB grit/flow analytics (which try to get at some of what makes trail activities potentially less valid as an input to VO2 max) could some how be used as a correction factor to allow contribution of both mountain biking and trail running to VO2 max?