find out total balance on MainAccount I don't understand receiving address usage at https://blockchain.info/api/api_receive
As I understand it it should work like this:

MainAccount
|--generatedReceivingAddress1 (then someone send btc and it will obtain e.g. 0.1btc)
|--generatedReceivingAddress2 (then someone send btc and it will obtain e.g. 0.2btc)
|--generatedReceivingAddress3 (then someone send btc and it will obtain e.g. 0.4btc)


Is in bitcoin ledger ability to find out:


which receiving addresses belong to MainAccount?
total balance of MainAccount (in this case 0.7btc)?


Many thanks
Blockchain.info is implementing bip32 and bip39, to understand how they're work you should read more about them.

https://github.com/bitcoin/bips/blob/master/bip-0032.mediawiki
https://github.com/bitcoin/bips/blob/master/bip-0039.mediawiki

Simple Explanation: 

When you create a wallet they create a mnemonic phrase (Bip39) and then they create your bitcoin seed using that mnemonic phrase.

that seed has a private key (xprv,zprv) and public key (xpub,zpub) for each derivation path, changing the path giving you more keys that belong to the same mnemonic phrase.

for example "MainAccount" in here has the following path:

m/44'/0'/0'/


When you create another "account" in your blockchain.info wallet they increase the last 0 to get new keys for the same mnemonic phrase.

Now we can derive addresses using public key (xpub,zpub) or private key (xprv,zprv).

which receiving addresses belong to MainAccount?

Every address has been derived from your "MainAccount" keys is belongs to that account.

total balance of MainAccount (in this case 0.7btc)?

They check each address balance belongs to MainAccount to get total balance.