Somehow I never played Diamond/Pearl so I am trying to avoid spoilers, which means trying to figure it out on release day from google results filled with lets-plays is problematic.
In map there is Location Info view, which seems to highlight locations based on... something.
Filters are:
Build in Guidebook of course contains no information for that feature, and no NPC said anything yet.
So... what's those filters for?
The four icons shown on the location info represents 2 types of trees you can find in the Sinnoh region. This legend is new to Brilliant Diamond and Shining Pearl, which explains why you couldn't find anything in the original guidebook:
Fruit Icon
The icon represents locations of berry trees
White Tree
This icon represents an empty honey tree. You can visit these trees to lather them with honey to attract Pokemon
Brown Tree
This icon represents a honey tree that has been lathered with honey to begin attracting Pokemon
Shiny Tree
This icon represents a honey tree that has a Pokemon that you can encounter when you interact with the tree.
