If you've ever wondered why your dwarves don't dig things out in the most logical manner, why they choose to prefer to go all the way to the topmost and leftmost corner of the map and dig instead of the places closest to them, or why they spend all their time chipping away at a project you've designated but don't immediately need instead of more pressing concerns, then this post is for you.
If you want to know the inner workings of how dwarves choose which tiles to dig, gather plants, channel, fell trees, or any other designation, then this post is also for you.
I've done a little observation on how dwarves choose designations. This is what I've found so far.
1. The map is divided into 16 x 16 squares. Dwarves will choose the leftmost, topmost, and deepest block that contains a designation. Note that they select the tile in that order.
2. Within the 16 x 16 block, dwarves choose the leftmost, then topmost tile to work.
3. If there are designations that meet this criteria but dwarves can't reach, the dwarves choose the first tile available that meets the above two criteria.
4. When dwarves finish working a designation, they will randomly select an adjacent designation. If there's a pattern to this selection, I have yet to see it.
5. If there's no adjacent designation, dwarves return to step 1.
This is list is incomplete, because there a few instances where my goblins (minor modding) don't follow this pattern. For example, I'm not entirely sure why my miners choose to work my pumping shaft designations from bottom to top instead of my magma duct in totality, which is below them. It's likely that there's an additional searching step that I missed, or one of the steps is out of order. I'll look into it some more and hopefully I'll be able to provide a complete picture.
But this is how your dwarves select designations, for the most part.