Having bought many pairs of shoes, I have loads of them which I currently do not use yet. Nevertheless, I would like to keep all my pairs of running shoes in the "Gear" section. However, since I have so many of them, it gets very impractical to select which shoes were used for any given activity from a very long selection list.
For my purposes, it would be helpful to have another category "New Shoes" or "Unused Shoes," or whatever it would be called, which would be akin to the category "Retired Shoes" currently existing. If I had any shoes which I do not want to use for now, I could move them simply into "New Shoes", and they would not show on the list of all the shoes when selecting shoes for any given activity.
Now, this implementation would be sufficient for me. However, the idea might be further developed to accomodate different needs of users. For example, users might be given an option to create their own categories of shoes. Someone, for instance, apart from having "Retired Shoes" and "New Shoes," might want to hide some of her used shoes which are not retired yet. And so she might want to create another category "Hidden shoes," which again would work just like "Retired Shoes" in preventing all the shoes listed there from showing on the list of shoes selection for activities.
To make it maximally configurable, the ability to create a new category of shoes could come with the option to determine whether the shoes belonging to the category should be visible on the list of shoes selection for activities. The reason is that someone might want to categorize shoes according to different activities, or according to the workout type (e.g., easy, tempo, race), or according to their color etc, in which case it would be desirable that the shoes under some of the custom categories still show on the list of shoes selection.