The process of mapping a listing to a specific category can be time consuming, especially if you sell on a number of marketplaces. It can get even trickier if the marketplaces use different languages. For each marketplaces as a seller you will need to locate the right category, pick its category id (node id, genre id... depending on the marketplace lingo) and put this id into your product feed before you will submit your feed to the marketplace.
In the case of some items, there is an obvious choice because a category exists that fits your item perfectly. If you have a 'Breitling Avenger Seawolf watch," for instance, the obvious choice for category is pretty specific, " Jewelry & Watches > Watches, Parts & Accessories > Wristwatches" However, in another example, a ladies jacket may fit well in "Women's Clothing and Accessories> Blazers and Jackets" but it may also qualify as "Women's Clothing and Accessories> Outerwear" in some climates.
Should you pick the category that is most common for this type of item? Should you choose the category where it appears less commonly or perhaps both if possible?
The trade off is this; in a category where the item is less common, the item will stand out. However, its exposure is likely to be limited to the audience most likely to be looking for it.
Placement in the more popular category assures that you can take advantage of human intuition and habit of how buyers look for the item first (and usually only) in the most common category. Putting the item in the more common category clearly exposes the item to a wider audience. So, failure to list in the most common category will also assure your best audience misses your offering.
Moreover if you use facets / item specifics / attributes which are used by marketplaces' navigation your listing has even higher chance of discovery if placed in a more common category.
And how about choosing a category which is outright wrong, but smaller and with a high chance to clearly stand out? Here you should consider that all marketplaces will enforce listing order and you will be risking remapping or removal of your listing at best or more severe sanctions if such actions are repeatedly discovered.
The last question you must ask yourself is whether or not it is worth the additional investment to use both, if the marketplaces allows it. To some degree it will take experimentation to determine what works and what doesn't. So, experiment and closely monitor the gained exposure (and hopefully higher price received) from the additional expense of listing in two categories. This will help you determine the "breakpoint," that place where it is most practical to decide to take on the extra expense to list in both categories, because the additional item exposure is worth the additional expense.
Needless to say that mapping significant quantities of listing to numerous marketplaces requires considerable amount of time and unfortunately this process never ends as new items are constantly introduced into the marketplace. To make it more challenging marketplaces take initiative and change their category structure from time to time, forcing the seller to remap whole segments of their inventory.