There are actually five attributes that digimon's have, Unidentified, Neutral, Data, Virus, and Vaccine. In Digimon Master Online, these attributes affected each other.
Red arrow means what is strong againts. For example, Data is strong againts Vaccine, and normal is weak againts all attribute. This cycle of attributes is very important. When you fight wild digimon which attribute is weak againts your digimon, you can take less damage and do more damage up to 50% damage difference. That makes you can fight wild digimon with 4 levels above your digimon's level without feeling overwhelmed at all.