Semi-auto has your character automatically running towards the enemy when you attack to ensure it hits whereas manual will have you attack regardless of whether or not you're close enough to the enemy, meaning that you could be swinging your sword at air.
"That's because the player keeps mashing the button!" -Deadpool after slapping Wolverine repeatedly in "Deadpool: The Game"
Assuming it works like every other tales, semi auto causes you to run up to the enemy before attacking if you hit an attack whereas manual just swings your weapon wherever you are if the target enemy is 20 feet away. I highly recommend manual as semi auto has tended to get confused with its pathing during a huge cluster of enemies in other 3D tales.
it should have to do with character control if its like past tales games.(meaning it should deal with enemy targeting and being able to free run at least, there may be more it does now though). manual auto targets enemies and restricts free run and semi-auto allowed free run and let you self target enemies more, it still targeted for you but you could change enemies manually if you wanted.
Semi Auto will automatically position your character when you attack or use Artes. This is useful as it will line you up with the enemy even after you're facing the wrong way or your Arte has some difficult hitbox.
Manual makes it so you control everything. You'll have to position yourself accordingly to get close to attack or make sure you face the right way during attacks. Jumping is achieved by pressing Up, whereas Semi Auto is done by Guarding+Up.
Just because it says Auto in it, doesn't make Semi Auto bad. They're both used for different things in their own rights.
~If you sprinkle when you tinkle~ ~Be a sweetie and wipe the seaty~