A hive fleet will contain a number of different species gene-spliced in quantity by its Dominatrix and Norn Queens. Each species is a kind of creature chosen from the basic Tyranid list in this book. By selecting it you are deciding to have that creature available on your hive fleet list. Because only so many eggs can be laid in a day, you are limited in the total number of different species you can include. You can choose to have up to a maximum of twelve different species in your hive fleet list.
For example, I decide to select the following species for my own swarm, hailing from Hive Fleet Arachnia.
Hive Tyrants Tyranid Warriors Lictors Hormagaunts Termagants Ripper Swarms Biovores Carnifexes
This makes a total of eight different species. When I pick an army to represent a swarm from Hive Fleet Arachnia I can only use species in the army from the list above. If you are asking yourself why would I limit my species, read Mutant Creatures below.
Errata — Chapter Approved: Codex: Tyranids Q&A (v2.0): Creating your own hive fleet fully replaces the thirteen broods printed in the Codex's own army list section — from then on, all of your army selections are drawn from your custom hive fleet species list instead. For creatures like Warriors that can already vary their weapons within a brood as standard, giving different broods different weapons (with identical biomorphs) does not create a new species — a new species requires different biomorphs. For creatures that can't normally vary their weapons within a brood (eg Gaunts, which must have identical weapons across the whole brood), changing the weapon does create a new species. Hive Mind powers chosen for a custom creature do not count towards its maximum number of biomorphs.