The PHP handler 5.5 FPM application (id=fpm) is not available on the destination server.
Instead, Plesk will use the following PHP handler: 5.5.38 FPM application served by Apache (id=plesk-php55-fpm)
PHP version that is installed on a source server that domain is using (PHP by OS vendor 5.5 FPM) is not installed on a target server. That's why Plesk Migrator suggested to use installed version: PHP by Plesk 5.5 FPM
This warning can ignored. If it is important that domain uses PHP by OS vendor 5.5 FPM, install it in Tools and Settings > Updates and Upgrades