a) Attribute mapping is depends upon the categories that you have mapped.
b) In which some attribute are "required" and some are not. If the attribute is "required" then you have to map it.
c) Attribute mapping is like, in which you have to map your Shopify variant options with Sears options.
For example, Sears has given the attribute in "Clothing|Men's Apparel|Men's Big & Tall|Men's Big & Tall Shirts ". Then if you have assigned your products in the same category and you have the variations in the products then you have to map the variant attribute with sears attributes, Like Shopify color with Sears color