On all of the Carnival Corporation lines, if you do not pull your tips, the amount is dispersed among the waitstaff, cabin steward, headwaiter and a small amount (about .50/day) goes to the behind the scenes staff (galley, laundry).
If you pull your tips and "decide" to tip in person then all of the money you give to an individual must go into the tip pool. If you do not pull your tips and give extra then the individual can keep the extra money. On NCL all extras are to be turned in to the pool.
The only major line that still uses the envelopes now is Royal Caribbean and it's lines. (Celebrity and Azamara)