The label to display when presenting the attribute to a user (e.g., "Title" or "Manufacturer").
The label is defined for the product, and is therefore not necessarily the same as the label that is defined in the characteristic set returned by another call like GetAttributesCS.
For GetProductSearchPage: If the attribute's label is "Keyword", it means you can enter the attribute's ID and a string value in the SearchAttributes node of GetProductSearchResults, and then eBay will search for the string against one or more attributes in the catalog. For example, for strollers, GetProductSearchPage only returns a Keyword attribute instead of separate Brand, Product Type, and Model attributes (for the US site). So, you can use the single Keyword attribute to search against all of those attributes.