Lojban does not have punctuation marks as such: the denpa bu and the slaka bu are really a part of the alphabet. Other languages, however, use punctuation marks extensively. As yet, Lojban does not have any words for these punctuation marks, but a mechanism exists for devising them: the cmavo “lau” of selma'o LAU. “lau” must always be followed by a BY word; the interpretation of the BY word is changed from a lerfu to a punctuation mark. Typically, this BY word would be a name or brivla with a “bu” suffix.
Why is “lau” necessary at all? Why not just use a “bu”-marked word and announce that it is always to be interpreted as a punctuation mark? Primarily to avoid ambiguity. The “bu” mechanism is extremely open-ended, and it is easy for Lojban users to make up “bu” words without bothering to explain what they mean. Using the “lau” cmavo flags at least the most important of such nonce lerfu words as having a special function: punctuation. (Exactly the same argument applies to the use of “zai” to signal an alphabet shift or “ce'a” to signal a font shift.)
Since different alphabets require different punctuation marks, the interpretation of a “lau”-marked lerfu word is affected by the current alphabet shift and the current font shift.