A Monster Tablet (usually just called a Slate) is a man-made mystery disc that has ancient inscriptions of the Pendant people on it. They used these inscriptions to mark which type of monster was inside (unlike Mystery Discs which all look alike) for easy storage and regeneration. Many of the characters are similar to the Monster Alphabet.
Tablet regeneration is introduced in Monster Rancher Advance and uses the unique format of the Gameboy games that do not allow you to unlock cartridges as monsters (like the other gaming platforms that can use CDs). You must put in coded words that are read by the Shrine master to unlock monsters.