Data sheet: pin configuration (make sure the PCB design is compatible with that)
there's different ordering code for one single microcontroller, coming with different property, ex: package, temperature tolerance. (For tap and reel, it's just a different final packaging method, it's on a tape rather than on a tray, only important if I'm assembling it by machine)
PBC design: trace size: 8/10 mil is by default, can go higher for high current part, ex: power source (10-15 mil can tolerate 1 Amp) via size: 30 mil big, and 15 mil hole is more optimal for production, than the smaller default one </aside>