Conveyor-belt levels

Conveyor-Belt Levels are levels where all the seed packets you receive come on a Conveyor Belt, with no cost in Sun. For this reason, you never receive Sunflowers or Sun-shrooms on these levels.

Levels
The final levels of the stages in Adventure Mode (Day, Pool, Fog, etc) (e.g. 1-10, 2-10, etc.) are Conveyor-Belt Levels, as well as the fifth level of every stage, except level 2-5, which is Whack a Zombie and level 4-5, which is a Vasebreaker level (these are puzzles).

Also, the Mini-games Wall-nut Bowling, Invisighoul, Big Trouble Little Zombie, Portal Combat, Column Like You See 'Em, Wall-nut Bowling 2, and Dr. Zomboss's Revenge are Conveyor-Belt Levels.

It's Raining Seeds and Slot Machine are not Conveyor-Belt Levels, although they have similar structures.

Strategy
Your strategy should be different, depending on the level, but remember to save up plants when you don't need them in case some of your other plants are eaten. Also, remember to plant at the back of the Lawn if you have enough range, as you don't need room for sun-producing plants.

Trivia

 * Sunflowers, Garlics, Umbrella Leaves, Marigolds and Planterns never come on Conveyor-Belt Levels, although Marigolds, Garlics and Planterns can fall from the sky.
 * You never receive Upgrade Plants or the Imitater, most likely because it wouldn't make sense to have them if you hadn't decided to purchase them yet.