All bullet trajectories are in an arc, not flat, dropping just after leaving the barrel. But you can use the word flatter in comparison of other cartridges. The flatter a bullet shoots, the less velocity it sheds, and retains more downrange energy and less drop. This is most desireable.

You mention some of the Nosler bullets: Partition bullets have just that, a copper division in the middle of the bullet so when the front half of lead expands or is broken away on impact, the solid lead filled bottom half of the bullet can still punch through. Good for use on larger animals or shots on bone and where deeper penetration is desired.

The Ballistic tip uses a poycarbonate tip to protect from deformity and give better aerodynamics to it. These bullets use a more fragile jacket without the partition to give more violent mushrooming, the plastic tip of the bullet is driven into the lead core to initiate expansion. Used for smaller thin skinned animals, when bone is hit, they tend to blow up in a big way. Lots of people I know who hunt deer, swear by these things for the gaping holes they create.

All bullet makers produce some sort of premium style bullets in popular weights and calibers to mimic what Nosler has done with their line.

30-06 is Springfield 30-06, named for the rifle for which it was originally chambered in. If you encounter something like 30-06 Improved, that's different from a 30-06 Springfield.