Digivolving MechanicsMost of this was already discovered, but I'm adding a bit to that.
The process is split into several steps. First, a digimon reaches maturity, then the game looks if the conditions of a digimon it can digivolve to are met and finally, the game picks the "best" or "most fitting" digivolution.
MaturityMaturity is measured in hours and resets upon digivolution. After a digimon reaches the necessary maturity, it will digivolve as soon as any conditions are met.
Baby 1 to Baby 2: 6 hours
Baby 2 to Rookie: 24 hours
Rookie to Champion: 72 hours (after 96 to Numemon)
Champion to Ultimate: 144 hours
ConditionsAs you may know, there are three main conditions and several bonus conditions for any digimon.
The main conditions are:
Your stats need to be higher than or equal to the required stats. (This is different when digivolving to Rookies)
Your care mistakes must be below or above (depending on digimon) a specified value.
Your weight needs to be within +-5 of the required weight.
The bonus conditions are: everything else.
To be even able to digivolve, you need to meet 3 main conditions of a digimon or 2 main and 1 bonus condition.
Digivolution FitnessMore often than not, you'll fulfill the conditions of more than one digivolution (most famously: If your Agumon fulfills Greymon's conditions, it will
automatically fulfill Centarumon's), so the game has a way of determining which digivolution 'fits' more.
The most important thing here are the stats for which there are requirements of the digimon you can digivolve to, but not the required values. That was a mouthful.
For example, Greymon requires certain values for Offense, Defense, Speed and Brains.
Centarumon only requires certain values for Brains, etc.
What you do with this is look at what the stats are, and then look at what values you have for these stats (Note: divide HP/MP by 10 and round down). Then, you calculate the average (and round down) and this is the fitness score. The digimon with the highest score wins and your partner will digivolve into that.
Example:
You have have an Agumon.
Stats: 800 HP, 600 MP, 102 Offense, 101 Defense, 102 Speed, 102 Brains
CM: 0 Care Mistakes
Weight: 30
Misc: 0 Battles
Looking at the conditions over at:
http://www.gamefaqs.com/ps/913684-digimon-world/faqs/64257
You'll see that you meet the conditions of: Greymon, Tyrannomon (CM, Weight, Battles) and Centarumon.
Greymon has requirements for Offense, Defense, Speed and Brains. Our values are 102, 101, 102 and 102. The average is (102+101+102+102)/4 = 101,75 and since we have to round down:
101Tyrannomon has requirements for HP and Defense. Our values are 80 (remember: divide HP by 10) and 101. The average is
90Centarumon has requirements only for Brains. Our value (and average) is
102Centarumon has the highest score and will therefore be selected. If only Offense was 5 points higher*, we'd get Greymon.
*Strictly speaking, we only need Offense to be 1 points higher. With 103/101/102/102, Greymon would have a score of 102. The game checks the digimon in a certain order and for one digimon to beat another, it needs to have a
higher score. Greymon is checked before Centarumon, so Centarumon'd lose.
"Naturally, I knows the hacker" - Kid, Ever17