In certain committees, there may be particular countries or positions that must be filled by an experienced delegate for the committee to function well. (Usually only a handful, if any!) In those few cases, the Secretariat will select the most meritorious applicant for the position, taking into account both past Model UN experience and their responses to the questions on the application form.
Otherwise, the vast majority of positions will be allocated so that as many people as possible get at least one of their preferences. Beyond that criteria, we attempt to give people as high a preference as possible. We strongly recommend considering all of your preferences carefully, not just your #1, so that we can give you a position you will love! Participants who make it difficult to allocate them a position (e.g. by repeating a single preference over and over again, or only preferencing highly popular countries) may miss out on being given a preference, if that has to occur.