There are many types of doll that serve all kinds of roles, though it is likely that any of the dolls can be utilized for any purpose with a software patch as Doll AI's are specifically mentioned as advanced enough to sustain emotion and self awareness.
Despite this, it is also implied that they do not have as much rights as a human, though there are anti-discrimination laws (Ep0 - Ch8)
Dolls are often seen serving as ensigns on the bridge of larger warships, allowing the Captain and other bridge staff more leeway and focus with their tasks. Depending on viewpoint, this leads to either an amusing or unfortunate drop in discipline.
Dolls also serve as personal secretaries, and the bonus bundle of relationship with the boss included alongside their salary. Secretorial Dolls can be converted into combat units within minutes, thusly, they most likely serve as security on the side, though they are probably too expensive, too high maintenance and too power consumptive to mass produce battle Dolls to replace human soldiers.
There are organisations though, that aim to mass produce Dolls until there is no need for humans to fight on the front line, such as Anne's Project for Perpetual Peace organisation, though it is derided by critics as "Project, Machine Society" by those who oppose mass automation of warfare, which could easily see humans made redundant.
Dolls are classified by generation, then series. Each series appears identical externally (Ep0 - Ch8), with an alphabetical prefix and a numerical suffix as a name.
A notable Doll is A-10, a 'Core' equipped Anti - High Rank Unit. The pinnacle of combat Doll models, with a shield equivalent to a Type 2, and super high DC particle emission and a super strong exoskeleton, it's stated by her creator Dr. Thor as the only combat unit aside from Warships and Knights that can fight against High Rank Beasts.