According to Coinbase, the Wells notice related to "aspects of the company's exchange, our staking service Coinbase Earn, and Coinbase Wallet". It's not terribly surprising that the SEC might have Coinbase Earn in its crosshairs, as it has recently taken action against similar products, such as Kraken's staking service. In the wake of the action against Kraken, Coinbase seemed to try to pre-empt SEC arguments by sending an email to customers emphasizing things like "You earn rewards from the protocol, not Coinbase". It doesn't look like this has shifted the SEC's thoughts much, though.
This should be an interesting saga to watch, partly because Coinbase has expressed willingness in the past to go head to head with the SEC.