Church Slavonic is written in three broad hands, and knowing which one a manuscript uses places it roughly in time and tells a reader what to expect on the page.
Ustav is the formal book hand of the earliest period. The letters stand upright and separate, evenly spaced, drawn slowly with an even stroke, and the page is generous with parchment. It is a hand for scripture and liturgy, made to be read aloud from a lectern, and it is the hand the Cyrillic letterforms were designed for.
Poluustav, the half ustav, comes into use from the fourteenth century. It is smaller, quicker and more compressed, it tolerates sloping and irregular letters, and it makes heavy use of abbreviation and of superscript letters written above the line. It is a hand for producing more books with less material, and it is the hand the first printers took as the model for their types, which is why printed Church Slavonic still looks like a fourteenth century compromise rather than an eleventh century book.
Skoropis is the cursive chancery hand, joined, rapid and often very difficult, used for charters, accounts, letters and administration rather than for liturgy. It develops its own letterforms, some of them barely recognisable as relatives of the book shapes.
The three overlap rather than replace one another. A single scriptorium could produce all three in a year, choosing the hand by the job in front of it.