AOR Summary:
Regiment of Foot: this is a generic type of infantry unit. It is recruitable in almost any type of European province, as long as the correct buildings are present. It is not a great deal in term of cost to performance, but it is not limited in numbers.
"Named" Regiments of Foot: these regiments are recruitable in a single province, and always in a limited number. The numerical limits can vary quite a bit -- from 16 or 18 for France and Russia, to 6 for the smaller minors. Majors can recruit at least 12 named line from their capital province. Some majors, notably Britain and Austria can recruit multiple types of line (Britain can recruit Scots and Irish if they build the right buildings; Austria can recruit Hungarian).
These regiments are always a better deal -- even if their quality isn't a lot better than the generic Regiment of Foot, they will cost less.
Here's the best part: any European power that conquers a recruiting province can then recruit those line troops in their own armies. The new units will also all keep their old uniforms.
Please note that the quality of line units does not reflect the mod designers' opinions on the quality of national or ethnic groups! Rather is a reflection of training, doctrinal usage and societal factors. Any well-led, well-paid and well-trained soldier will be a good soldier.
Germanic Line
These Regiments of Foot, while not quite to the Prussian standard, are raised and trained on the Prussian model. They value good leadership, a quick rate of fire and strong mutual support, but they are a little more expensive to raise.
Bavaria, Saxony, Westphalia, Wurttemburg, Hannover
Italian Line
These Regiments of Foot are made of strong willed and brave individuals -- but they are poorly led, and not trained to the highest standard. Their rate of fire is a little slower than average, but they defend tenaciously. Unfortunately, due to mediocre officers and NCOs, their morale is a little lower. They are a bit less expensive to raise.
Genoa, Papal States, Savoy, Venice
Scots and Irish Line
These Regiments of Foot are trained on the British model, and they value strong mutual support and high rate of fire. The rough upbringing of the Scottish and Irish soldiers has made them deadly in hand-to-hand combat. They are more expensive to train.
Scotland, Ireland (with military buildings)
Others
Hungarian Line, like Austrian, is poorly trained and led. Hungarian soldiers do not lack bravery, however, and have a devestating charge. They are cheaper to raise.
Hungary
Danish Line is trained to a high standard, and led well -- they must be, in order to resist the Swedish advances accross the Baltic. Danish Line has has a very strong charge and is dangerous in melee.
Denmark
Polish Line has a slower rate of fire, a strong charge, but is not well led. They are a little cheaper to raise.
Poland
Portuguese Line has been trained along a British model. They are strong defensively, but not well led.
Portugal
Dutch Line is all around good, but does not excel (or lag) in any one area.
The Netherlands
Walloon Guards are well trained, highly motivated soldiers. They are as good as any Guard Infantry in Europe (and as expensive to raise and train!). They are recruitable in Flanders with an Army Staff College.
Flanders