The field corresponds to false for mainnet, true for testnet, and false for regtest. You can think of it as a flag that tells you whether the current network is testnet.

Why's it named like that?

To get people to not use it, and because getinfo and getmininginfo still use it:

jtimon commented:

Yeah, comparing to the testnet id was very readable so it's the new flag. The difference is that when people see "NetworkID() == something" they think they can use that too, while FlagWithLongNameIncludingTheWordDeprecated() is not as likely to be used or that's my hope. @laanwj explained the deprecation thing, but basically, RPCs can just use bip70's networkidstring() For the string, I was going to create another flag but @laanwj wants to keep chainparams clean of GUI-related things. Of course comparing to an enumerator is better than comparing to a string, again, the goal is that people don't see the comparation to the enumeration and think they can call CChainParams::NetworkID(), hopefully the comparison with a string will make people use that approach less, but we need the string for BIP70 anyway.

(Source.)