The User Activated Hard Fork (UAHF) is a proposal to increase the Bitcoin block size scheduled to activate on August 1. The UAHF is incompatible with the current Bitcoin ruleset and will create a separate blockchain. Should UAHF activate on August 1, Coinbase will not support the new blockchain or its associated coin.
The User Activated Soft Fork (UASF) is a proposal to adopt Segregated Witness on the Bitcoin blockchain and could result in network instability. It is scheduled to activate at the same time as the UAHF.