Standard Notes can be used totally offline without an account, and without an internet connection.
When using Standard Notes without signing up for a free sync account, any data you create will remain only on the local machine, and will not be synced. There are a few things to note in this scenario:
When using Standard Notes without an account, you'll need an alternative source of encryption, since no password has been set. To enable encryption when using the application offline, set up an Application Passcode. You can do this via the Security menu. Once a passcode is enabled, the same level of encryption will be used as if you were using the application with an account.
If you do use an account to keep your notes synced and encrypted, but encounter a scenario with limited network connectivity, Standard Notes will continue to function.
Any notes which have been previously synced to the device will be accessible without a connection. Any changes you make to notes while offline will be saved to the device. These changes will be synced to your account as soon as network connectivity is restored and the application is opened.