If miners activate SegWit prior to 1 August 2017 then BIP 148 will not be enforced. All users of Bitcoin will remain on the same chain no matter what client they use.
If miners do not activate SegWit then there may be a chain split. A chain split will mean that some users will see a different set of transactions than others.
If the majority of miners enforce BIP 148 then these chain splits will be temporary and eventually all clients will see the same chain and SegWit will activate for all SegWit compatible clients (0.13.1+ for Bitcoin Core).
If the majority of miners do not enforce BIP 148, users that enforce BIP 148 will diverge from users that do not enforce it.
If the majority of miners do start enforcing BIP 148 at a later date then the legacy chain (without BIP 148 enforcement) may be reorganized once the BIP 148 chain has more work.
The users who are running BIP 148 would be undisturbed, but users running legacy clients may see a large amount of history re-written and could lose funds. https://www.weusecoins.com/uasf-guide/
What could happen on 1 August 2017?
If miners activate SegWit prior to 1 August 2017 then BIP 148 will not be enforced. All users of Bitcoin will remain on the same chain no matter what client they use.
If miners do not activate SegWit then there may be a chain split. A chain split will mean that some users will see a different set of transactions than others.
If the majority of miners enforce BIP 148 then these chain splits will be temporary and eventually all clients will see the same chain and SegWit will activate for all SegWit compatible clients (0.13.1+ for Bitcoin Core).
If the majority of miners do not enforce BIP 148, users that enforce BIP 148 will diverge from users that do not enforce it.
If the majority of miners do start enforcing BIP 148 at a later date then the legacy chain (without BIP 148 enforcement) may be reorganized once the BIP 148 chain has more work.
The users who are running BIP 148 would be undisturbed, but users running legacy clients may see a large amount of history re-written and could lose funds.
https://www.weusecoins.com/uasf-guide/