Answers to questions. You know the drill!

How is history stored?

Chat history is stored in memory, and is never stored on disk.

How many messages are stored in history?

2048 messages are stored per channel, and 250 are shown automatically on channel join. More can be shown using the /histserv command. 256 private messages are saved per private message conversation.

How long are messages stored in history?

1 month.

What is an "always-on" client?

An "always-on" client is a function that allows clients that are disconnected to still receive messages, and appear as if they are online. It makes it possible to preserve channel user modes as well, and performs the same function as a bouncer such as ZNC.
All registered accounts are "always-on" by default.
If a client hasn't been used in 30 days, its always-on status will be removed until the client has returned. This is to avoid accounts that become abandoned from showing up in channels forever.

How do I register an account?

See this page.