The force organisation charts for the scenarios in Warhammer 40,000 often require specific units to be used, most commonly HQ and Troops. In order to satisfy this, your hive fleet species list must include at least one each of the following types: HQ, Troops, Elites and Heavy Support. Any swarm chosen from your hive fleet list must obey the force organisation requirements for the mission you are playing (ie, one HQ and two Troops in a Standard Mission).
Sentries In missions where the Sentries scenario special rules are used, Tyranids usually get eight Termagants to deploy as sentries. Now it is possible that your hive fleet list won't include Termagants, or even Gaunts of any description. Instead, when you have to deploy sentries, use a maximum of 60 points worth of creatures picked from your hive fleet list. All the creatures must be of the same type and must be from either the Troops or Elites sections. Ripper swarms may not be chosen as sentries.
Errata — Chapter Approved: Codex: Tyranids Q&A (v2.0): The minimum possible number of species in a hive fleet list is 3 — a Heavy Support species, a Troops species, and an Elite species that includes at least one Warrior species (which can be taken as HQ or Elites) is enough to satisfy all four required categories.