From the fourteenth century onward Slavonic scribes wrote increasingly in the half ustav hand, and one of its habits changes the appearance of the page completely: letters are lifted out of the line and written above it.
A superscript letter usually stands for the last consonant of a word or of a syllable, and it is typically covered by a small arc or a titlo to show that it belongs to the word below. Whole endings can be handled this way. The saving in space is real, but the deeper reason is speed, and the practice grew as demand for books outran the supply of parchment and of time.
A related mark is the paerok, a small hook standing where a reduced vowel has been left unwritten. Once the reduced vowels had stopped being pronounced in most positions, scribes stopped writing their letters, but the orthography still expected something there, and the paerok is the compromise: an acknowledgement that a letter is missing rather than a claim that it never existed. It is one of the clearest pieces of evidence for dating a sound change, because a scribe only needs a mark for an omission once the omission has become normal.
Both devices survived into printing. Church Slavonic type carries sorts for superscript letters and for the paerok, and a modern liturgical book still uses them, which means that a convention invented to save parchment in a fourteenth century scriptorium is still shaping the look of a page printed this year.