With Pokemon Sun and Moon, Gamefreak introduced an innocuous one-time-use hold item set called Seeds which would slightly boost the stats of the user when held in a certain terrain. The collective response surmounted to something along the lines of "oh, neat" and they were never regarded again.
And yet, hold items that activate on terrain have some pretty big implications for a game like Reborn, in which you'll fight on field effects more often than not. As such, the new seed items have gotten a bit of a facelift.
Like in Sun and Moon there are still only four seed items, but instead of being one for each field, all 37 of our field effects have been categorized into one of four types:
And the seeds have followed suit! Rather than seed items just working on one specific type of field, they'll activate on any field of their type. More than that, every seed+field combination produces, not only a stat boost, but a unique effect. Not all seeds are created equal! Some are fairly tame, but others heal you, or boost lots of stats in return for damaging you, or can change your type completely! Which Seed does what on which field is covered in your Field Notes app, so make sure to collect and check those to know what you're packin'. They say knowledge is power, but so is opportunism!
I invite you to play around and use them to the best of your abilities. Your opponents certainly will be too, after all.