As of September 2018 accounts are only supported by the CLI wallet and missing from GUI wallet.
Accounts are similar to subaccounts in your classic bank account. There is a very important difference though. In Monero funds don't really sit on accounts or public addresses. Public addresses are conceptually a gateway or a routing mechanism. Funds sit on transactions' unspent outputs. Thus, a single transaction can - in principle - aggregate and spend outputs from multiple addresses (and by extension from multiple accounts). The CLI or GUI wallet may not directly support creating such transactions for simplicity.
In short, think of accounts as a **soft** grouping of your funds.
0 | 1 | identifies the network and address type; [42](https://github.com/monero-project/monero/blob/784f7b07f05a645d43f62ed3a9cefda4b0c57825/src/cryptonote_config.h#L153) - main chain; [63](https://github.com/monero-project/monero/blob/784f7b07f05a645d43f62ed3a9cefda4b0c57825/src/cryptonote_config.h#L167) - test chain
* It is not recommended to sweep all the balances of subaddress to main address in a single transaction. That links the subaddresses together on the blockchain. However, this only concerns privacy against specific sender and the situation will never get worse than not using subaddresses in the first place. If you need to join funds while preserving maximum privacy do it with individual transactions (one per subaddress).
* Convenience labels are not preserved when recreating from seed.