But you can see the bottom line next to it, so it cannot be a 'A'. The 'A' letter doesn't have a bottom line.

I believe that it's SEALING RINOA.
If Squall never saved her, she would be pro-longing her life.
If someone goes back to the past and stopped Squall from saving Rinoa, she could live for many years and become Ultimecia.
It shows how dedicated Esthar really is. When they analysed Rinoa, they acknowledged the potential in her power.

I don't think Esthar seal sorcessess so that they wouldn't harm the Estharians. It's something else. They wanted to use sorcessess as weapons to defeat something in the future. Maybe X?