Mainnet is the main blockchain where Decred has real-world economic value. dcrd, dcrwallet, Decrediton and all other Decred applications will start in mainnet mode by default unless the user has specified another network using config files or CLI flags.
It is not recommended to test new or experimental features on mainnet. Testing on testnet or simnet is more appropriate. These networks carry no economic value, so testing can be performed free from risk or cost.