The Adv says no Late Fee or Penal Interest Rate.

Normally if you pay the minimum by the due date, there is;  
- Normal APR applicable.  

If you don't make a payment or make the payment after the due date or don't make the minimum payment, there is;  
- Normal APR  
- Fixed Penalty Fee for late payment  
- Additional Penal Rate of Interest


What the Citi Card is offering is;  
If you don't make a payment or make the payment after the due date or don't make the minimum payment, there is;  
- Normal APR  
Even the notmal APR is being waived for first 18 months on Citi or 14 months on Discoverer.

Although the fine prints are not available; it could very well be quite a few things;  
- Cap on the Card Limit. If you get a card with a limit of $500. It doesn't cost much of the Bank to recover this over a period of 18 months.  
- There are no reward points or other cashback as available maybe with other cards. This would save the Card company some money.  
- The fine print could also be that only initial months purchases will get 0% APR and unless these are settled, the new purchases will attract higher APR.  
- The card may only be issued to someone with GREAT Credit history, the adv a mechanism to attract good customer who always repay on time and would never use the feature but want it just in case.  
- There could also be limits on which purchases enjoy 0% APR [ie min / max amount per tranaction, etc] in the fine prints.