and more... » her tattoo

Usually, when parents pass away they leave their children some sort of in heritance — money, a house, some collection or other. In Hawkeye's case, her father left his lifetime research, something that couldn't hand to just anyone, and trusted her to pass it on to someone she would consider worthy of that knowledge. The only problem was that research was encoded and carved on her daughter's back. Now that'd do the trick.

Hawkeye ended up granting that knowledge to one Roy Mustang. The delicate place in which the tattoo was drawn also ensured his father she wouldn't let just anyone take a look at it, but someone she completely trusted. In any case, it would have been hard to explain for someone that didn't know her.

Filled with guilt after the war, she would also ask Roy to erase it so no other flame alchemist would be born. He used the same power the tattoo taught him to carefully burn Hawkeye's back until the whole content would be irrecoverable. Before the flames did their job, the tattoo looked something like this:

Obviously, the alchemic array on Roy's gloves is derived from that drawing. It's, in fact, a simpler form of it. Presumably, a combination of the text, symbols and the arrangement allowed him to decypher the secret of Professor Hawkeye's fire manipulation alchemy. The text found in the tattoo is actually the Libera Me, a Roman Catholic responsory in our world. This type of chant in latin is sung before a burial, at the absolution of the dead. The complete text goes as follows:

Libera me, Domine, de morte aeterna, in die illa tremenda:
Quando caeli movendi sunt et terra.
Dum veneris judicare saeculum per ignem.
Tremens factus sum ego, et timeo, dum discussio venerit, atque ventura ira.
Quando caeli movendi sunt et terra.
Dies illa, dies irae, calamitatis et miseriae, dies magna et amara valde.
Dum veneris judicare saeculum per ignem.
Requiem aeternam dona eis, Domine: et lux perpetua luceat eis.

Which translates to:

Deliver me, O Lord, from death eternal on that fearful day,
when the heavens and the earth shall be moved,
when thou shalt come to judge the world by fire.
I am made to tremble, and I fear, till the judgment be upon us, and the coming wrath,
when the heavens and the earth shall be moved.
That day, day of wrath, calamity, and misery, day of great and exceeding bitterness,
when thou shalt come to judge the world by fire.
Rest eternal grant unto them, O Lord: and let light perpetual shine upon them.

A connection to fire is visible, just as you can see a salamander in the drawing — perhaps a reference to the mythical salamander, which makes its home in fires. Moreover, Paracelsus (the Renaissance physician, botanist, astrologer and alchemist) suggested that the salamander was the elemental of fire. Symbolism is aplenty, what remains in the shadows is Professor Hawkeye's true intentions behind his research, something we may never know.