To check your progress on this trophy open the Field Guide in your pause menu (
) and look at the top right corner. The trophy will pop when the Digimon Registered reads 300 and to have a Digimon be registered and increase your total you must have actually OWNED said Digimon at some point.
To compare it to Pokemon's Pokedex:
Digimon Encountered = Pokemon Seen
Digimon Registered = Pokemon Caught
Since Digimon don't have one linear evolution tree like Pokemon do this trophy is much easier than it seems.
For example Koromon can Digivolve into 6 different Rookie level Digimon so we'll follow the Agumon route from the anime.
Koromon > Agumon > Greymon > MetalGreymon > WarGreymon > Omnimon
as you can see this Digivolution tree has a total of 6 forms from Koromon to Omnimon so every time you get a new Digimon
for the first time it counts as one.
Let's say I used the Digimon Black Market and bought my first Greymon and then I immediately went to the DigiLab and used the DigiConvert option to make my first Koromon.
If I Digivolved Koromon to Agumon, then that same Agumon to Greymon, then that same Greymon to MetalGreymon that would be 4 new registries
HOWEVER because I bought that Greymon from the black market it doesn't count as a new registry and therefore I'm only given credit for 3.
You can Digivolve your lower levels to their Mega forms, then De-Digivolve those Mega forms to an unregistered Ultimate form provided you have discovered (encountered) that Digimon.
Any Digimon you
can't De-Digivolve to will be a silhouette with ??? for a name and will say "De-Digivolution Not Possible..."
Digimon you
can De-Digivolve into will just be a faded image with a proper name and will say "De-Digivolution Possible!"
The trophy will pop when you open your guide when there's 300 registered Digimon.