If you select mainnet, you will transact "real" ETH (those who are worth $470 and can be deposited in exchanges, used on ICOs, etc...);
If you select testnet, you will transact "fake" ETH (like the name says, ETH meant for testing, which isn't worth anything and has its own blockchain). Basically, play money.
so the testnet part can not be used at all to transact?