Migration can be canceled, but this should only be done in extreme circumstances when the source is no longer available. Once migration has been canceled, it cannot be resumed. The primary purpose is to allow migration to complete when there are uncorrectable errors on the source. If the complete filesystem has finished migration except for a few files or directories, and there is no way to correct these errors (i.e. the source is permanently broken), then canceling the migration will allow the local filesystem to resume status as a 'normal' filesystem.
To cancel migration in the BUI, click the close icon next to the progress bar in the left column of the share in question. In the CLI, migrate to the shadow node beneath the filesystem and run the cancel command.