BIS is an acronym standing for: Acronyms and initialisms are abbreviations formed from the initial letter or letters of words, such as NATO and XHTML, and are pronounced in a way that is distinct from the full pronunciation of what the letters stand for. ...
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We remind that.bi stands for Republic of Burundi and for nothing else." This is apparently intended to discourage the "repurposing" of.bi as has been done with other ccTLDs (such as.tv for "television" and.la for Los Angeles), or the use in "domain hacks" where the TLD becomes part of a word or phrase (as in del.icio.us).
Use of "bi" in the sense of bisexual is likely the sort of thing they are aiming to prevent, though it is unclear whether there have been any actual uses of the ccTLD in this manner.
.bi is one of the minority of ccTLDs which has signed a Memorandum of Understanding (MoU) with ICANN, something ICANN has attempted with limited success to get all ccTLD registries to do.