Master Seal/Secondary Seal

[SuperZambezi]

What do these do exactly? And when is the best time to use them? And on who?

I'm on Ch. 14 so I feel like I should have used some by now.

 

[Yoshiman222]

What do these do exactly? And when is the best time to use them? And on who?

I'm on Ch. 14 so I feel like I should have used some by now.

Master Seals upgrade your character class. I suggest you use them when you reach Lv. 20 with any non-promoted characters.

Second Seals reclass the unit to one of the few that they are able to change into (based on their personality). Your unit gains access to all classes to his/her gender, so there's lots of choice there. That I suggest anytime if you want to teach the character skills from another set. For unpromoted classes, best used at Lv. 10 I believe (though it doesn't really matter much) and for promoted classes, best used at Lv. 15 so they can learn all the skills before changing.
You can also demote a promoted class to an unpromoted class to learn skills there too, but I recommend that for post-game (or if you're just going to grind against really weak enemies, which is a waste of time).
Oh, and finally, if someone is level 20, you can have them re-start their level-up period all over again by using a Second Seal, useful for promoted classes at Lv. 20 if you don't want them to reclass (instead of Lv. 15 like I suggested if you want to reclass).

Overall, if you're like me, I won't use Second Seals too often throughout the game until you beat it or you hate the class someone has.

Oh, and who to use them on - Master Seals, everyone but Frederick, Libra, and other promoted classes you get of whom I've forgotten for now since they can't use it, and I highly suggest using the Second Seal on Donnel if you have him because the Villager class is the worst class ever. He can easily use his aptitude skill he learns from it to better use as a Mercenary or a Fighter.

Completely up to you whom to use them on and when.

Any chara can use either('cept Chrom).

Master Seal - Promotes 1st tier units lvl. 10 or higher to 2nd tier.

Second Seal - Reclasses 1st tier units lvl. 10 or higher. Ex.: Can change thief into mage.

Second Seal also works for 2nd tier lvl. 10 or higher.

Another function for Second Seal is to avert a units lvl. back to 1 once they've reached the max(lvl. 20).

Believe I did my promoting around Ch.13.

Mine's better.
And it can be used on Chrom, btw.
His promotion isn't tied to the storyline like other games.

 

[Yoshiman222]

Oh... Chrom can... WTH then.

He's lvl.18 and it said I couldn't... Whatever.

Missing out on the Great Lord then... he carries lances as well so you might need to grind or get some of those scroll items (forgot the name) to make him useful. It's good because now he can fight from a distance with spears.