Sorry, but how could this metric possibly be the second most popular KPI under the Retail trade classification? Yes it is an indicator of how efficiently you manage your inventory situation but frankly there are better measurements for that. There are many retail KPI’s that are commonly used much more frequently than this one.
stock not displayed to customers in fashion industry is not good stock,
unless some basic items you need to keep stock in storage not in showroom, exp. mass retail ( they have bog quantities each size/color ).
to evaluate this KPI you really need to know your product, and the supply chain process/time to replace.
if items are available in one day time frame why you need to buy more stock and put it in storage " attention to market demand changes"
so, it is a way to measure your stock level health :)