Note LANGUAGE_MULTIPLE was later removed in #1869292: Remove confusing "multiple" language from core
Drupal 7 used LANGUAGE_NONE for cases, when the user did not know which language to apply or the user knew there was no linguistic information assignable or when multiple languages could have been assigned. To separate these cases, we renamed the LANGUAGE_NONE constant to LANGUAGE_NOT_SPECIFIED (retaining the 'und' - undefined value for it). We also added LANGUAGE_NOT_APPLICABLE for the case when language information is not applicable at all and LANGUAGE_MULTIPLE for cases when a set of languages might be applied but the individual pieces cannot be marked as such (like a PDF file uploaded on one in Drupal).
User interfaces to assign these and better defaults to apply these to new content are being worked on in #1471432: Rework language_list(), let people use more special languages and #1471444: Review use of LANGUAGE_NOT_SPECIFIED vs. LANGUAGE_NOT_APPLICABLE vs. LANGUAGE_MULTIPLE.