Liblouis User's and Programmer's Manual
2.6.2.2 Letter indicator ¶
Some languages have special indicators for single letter emphasis. The letter indicator will be chosen over other indicators when the next character is emphasized, but not the characters thereafter.
In some situations the letter indicator is the only way to indicate emphasis. For instance when emphasis ends within the middle of a word, and a word indicator exists (see Word indicator), but no way to cancel it explicitly. In this case Liblouis will use the letter indicator for every emphasized character in the word.
A table is even allowed to define only a letter indicator and no word or permanent indicators (see Permanent indicator), in which case the letter indicator will be used for any emphasis. It is not very likely that there are braille systems that work this way, but this feature can be useful anyway.