Are meals classified by time slot or sequence?

It is a combination of time slot and sequence. The main meal of the day is dinner, which can be eaten at midday or evening, depending on circumstances.

dinner, n. 'The chief meal of the day, eaten originally, and still by the majority of people, about the middle of the day (cf. German Mittagsessen), but now, by the professional and fashionable classes, usually in the evening'

If dinner is in the evening, the midday meal becomes lunch, though traditionally it could be at anytime. The defining characteristic being that it was not the main meal.

If dinner is midday, the evening meal can be lunch, or tea if it a very light (possibly uncooked) meal. The concept with 'tea' is that the food is an accompaniment for the cup of tea.

There are regional variations though. Australians seem to see tea as quite a heavy meal in the evening, and I have been told that Jamaicans refer l the first meal of day as tea.

This is really regional and has changed during the ages. For example, the midday meal was always called "dinner" in the farming area I grew up in (sometimes you will still hear this, though it's dying out now), and the evening meal was called supper.

Then there's the Hobbit dining plan - breakfast, second breakfast, elevenses...

I don't think either is definitive. If I wake up jetlagged at 7pm, I can have breakfast cereal, in which case I'd say I'm having breakfast; or I can whatever everyone else is eating, in which case I'd say I'm having dinner. In both cases, the nature of the food consumed seems to determine what you call the meal (breakfasty things make it a breakfast, dinnery things make it a dinner). That said, the nature of the food also isn't decisive, as you can have breakfast cereal for dinner (as a friend of mine, growing up in a house without much food, was sometimes forced to do).