Rucking as an Activity

But the user wants to know an differentiate.

I couldn't agree more that a rucking activity or data field for added weight that works with runs, hikes, walks and more would be vital. 

There's always the notes / comment of the activity where you can enter the weight information you want to record?

The notes don't have a function to show how the weight adds to the workout Bitti

Ok, but what workout stats should it affect?

HR is measured already, and it increases with additional weight (as trux has mentioned). HR increase over resting HR is used to calculate energy expenditure over base metabolism, and HR also provides EPOC.

I do believe that additional weight advances muscle development, but that's not something that the watches can measure.

One of the things Garmin tries to do is keep track of exercise load and estimate recovery times. Without factoring in weight, recovery times will be underestimated, as additional weight doesn't increase heart rate quite as reliably as lets say walking faster. You can see this in action by recording a "strength" activity with weights, and then altering the weights. And even if you don't trust Garmin's metrics, you may want to have a proper record of your training, preferably in a more structured manner than free-form notes.

The reason it MUST be added as an activity is HR calculation is grossly inadequate measure when rucking. The watch is blind to the full athletic performance and sees me as a guy who just walked 4 miles, at about 15:30 min/mile with an insane heart rate...essentially I look like I am grossly out of shape when accounting for HR, time, and distance. The VO2 max calculation looks like I should be on  heart-lung machine, all because it cannot account anywhere in its calculation for the 45 pound plate in the ruck I had on my back. In reality I can run a 5k in about 22.5 minutes with no added weight, but the watch estimates I can do it eventually.

Wrong, the watch just sees you moving slower and your heart working harder to move slower.

Here is a great article to check out: calculator-online.net/.../