I have found this definition of Share of Wallet on Wikipedia:
Share of Wallet is the percentage (“share”) of a customer’s expenses (“of wallet”) for a product that goes to the firm selling the product. Different firms fight over the share they have of a customer’s wallet, all trying to get as much as possible. Typically, these different firms don’t sell the same but rather ancillary or complementary product.
Measuring your share of given wallets – and your performance in increasing it – is a multi-domain master data management exercise as you have to master both a 360° view of customers and a 360° view of products.
With customer master data you are forced to handle uniqueness (consolidate duplicates) of customers and handle hierarchies of customers, which is further explained in the post 360° Business Partner View.
With product master data you are not only forced to categorize your own products and handle hierarchies within, but you also need to adapt to external categorizations in order to getting access to external data available for spending probably on a high level for a segment of customers but sometimes even possible down to the single customer.
Location master data may be important here for geographical segmentations and identification.
My educated guess is that companies will increasing rely on having better data quality and master data management processes and infrastructure in order to measure precise shares of wallets and thereby gain advantages in a stiff competition rather than relying on gut feelings and best guesses.