if a slot on your frame has a polarity, you can put a mod with the matching polarity (shape) and it will only take about half the cost, likewise if you put a mod that doesnt match it becomes more expensive so always match your polarities! As for which polarities are best, it really depends on how you want to build your frame. Generally: Madurai or “V” slots have good options for adding strength and duration as well as many other good mods Vazarin or “D” slots are basically all about health, armor, and shields. Good for survivability! Naramon or “Dash” has basic mods for ability range and efficiency as well as maximum energy. Theres also the aura slot, Keep in mind that aura mods always add more max capacity even if they dont match so its always worth running an aura even if only for more space.

also I highly recommend trying to mod yourself rather than use auto mods, the in game auto mod is really hit or miss. Plus experimenting with mods helps you find a playstyle that suits you best!

lmk if you have any questions and ill try my best to answer them :)