Phonotactics deals with restrictions on permissible combinations of phonemes in a language. Specifically, phonotactics are the rules governing how phonemes can be combined and ordered within syllables and words. There are two types of gaps in phonotactic patterns: accidental gaps, which are words that could exist based on grammar rules but don't, and systematic gaps, where a sound or word is prohibited from existing according to the phonotactic rules of the language.

2.  Phonotactics is a branch of phonology that
deals with restrictions (limits) in a language
on the permissible combination of phonemes
 In other words, Phonotactics are the rules that
govern the combinations and ordering of
phonemes in a syllable or a word

3.  Accidental gaps:
A gap in the pattern, is a word or other form that does not exist in
some language but which would be permitted by the grammatical
rules of the language, e.g. vig/lig
 Systematic gaps:
A gap through which we conclude that the specific/desired
sound/words can or cannot be exist in the language. e.g. vsij/nsdl