What types of private keys can be stored using Steelwallet?

Steelwallet supports the BIP-39 standard which allows you to store 12,18 and 24 word seeds on your Steelwallet. 
The words on the BIP-39 word list are chosen so that there is only one possible word if you know the first four letters of that word. It is impossible that there are multiple words that start with the same four letters. 
That’s why you only need (and only can) punch the first 4 letters of word of your seed onto your Steelwallet.