Ek - refers to the 96-00 civic hatchbacks here in us. In japan the EK9, where it originally comes from is the Civic Type R.
Eg- refers to the 92-95 civic hatchback series
Ef- is the pre 92 hatchbacks and crx's (Ef 8)
All these terms where derived from the japanese chassis codes, because here in the us I think the codes are different. I dont think there is anythig like this for a 97 ex, so just call it a ex. I know the 99-00 si's are em1's...