Depending on the call and context, this value is either a name of an Item/Category/Variation Specific, or a Parts Compatibility name.
For the AddItem and AddFixedPriceItem families of calls: In the Item.ItemSpecifics context, this can be any name that the seller wants to use. However, to help buyers find items more easily, it is a good idea to try to use a recommended name when possible (see GetCategorySpecifics). You can't specify the same name twice within the same listing.
For the AddFixedPriceItem family of calls: In the VariationSpecifics context, this can be any name that the seller wants to use, unless the VariationsEnabled flag is false for the name in the GetCategorySpecifics response. For example, for some categories eBay may recommend that you only use "Brand" as a shared name at the Item level, not in variations.
For the AddFixedPriceItem family of calls: In the Compatibility.NameValueList context, this value is a motor vehicle aspect such as Year, Make, and Model. A Compatibility.NameValueList container is applicable for motor vehicle parts and accessories listings.
For GetCategorySpecifics: This is a recommended (popular) name to use for items in the specified category (e.g., "Brand" might be recommended, not "Manufacturer").
For PlaceOffer: Required if the item being purchased includes Item Variations.
Note: For mandatory and recommended item specifics that are returned in the GetCategorySpecifics call, the seller should pass in the name of these item specifics just as they are shown in the GetCategorySpecifics response. Similarly, for catalog-enabled categories, the seller should pass in the name of instance aspects just as they are shown in the GetCategorySpecifics response. Instance aspects are additional details unique to a specific item or listing that a seller can include along with the product aspects that are already defined in the eBay catalog product associated with the listing. Instance aspects common to many catalog-enabled categories include 'Bundle Description' and 'Modification Description'. As of August 30, 2018, eBay US sellers who ship to California are required to pass in a 'California Prop 65 Warning' item specific if the item in the listing contains one or more chemicals known to be harmful to human health. This item specific is passed at the listing level for both single-variation and multiple-variation listings. The 'California Prop 65 Warning' is considered an instance specific because it will be retained on the listing even when the seller lists using an eBay catalog product. The 'California Prop 65 Warning' may be applicable in most eBay US categories, including eBay Motors, Motors Parts & Accessories, and catalog-enabled categories. Note: If Brand and MPN (Manufacturer Part Number) are being used to identify product variations in a multiple-variation listing, the Brand must be specified at the item level (ItemSpecifics container) and the MPN for each product variation must be specified at the variation level (VariationSpecifics container). The Brand name must be the same for all variations within a single listing.