Setting Amazon Listing Categories
Amazon always requires a category (Browse Node ID in Amazon terminology) for all newly created listings (i.e. the UPC/EAN that you are using to create the listing with doesn't exist on Amazon).
If you are creating an 'offer' for an existing Amazon catalog product i.e. uploading your (price+inventory+shipping) for an existing product being sold, then a category is not required. In this case you are using the category that already exists for that ASIN so there is no need to set it.
Setting the Amazon category/browse node in Codisto LINQ
Scroll right on the Marketplace Listings view until you see the Category column.
To set a category for an individual product, click the cell on the category column for the relevant product (click near "none"). This will open the category selector (see below)
To set categories in bulk:
Filter down to a set of products to bulk apply using any of the filter (funnel) icons at the top of the columns. (Filtering is optional - you can apply to the whole product catalog if appropriate):
Use the top left check box to "select all" which will show the number of products you are about to update.
Then click "Edit" on the top row of the category column:
This will open the category selector (note the view is different for different Amazon marketplaces):
Amazon Category Selection Dialog
Click the icon to expand a category to expose it's child categories. Repeat this process until you see the leaf categories, which have a radio button selection. Click a leaf category to select it, then click OK.
Amazon Category Selection Dialog showing leaf categories
Save & Send the changes to Amazon
Do this by clicking white Save button at top