Only Runes from Tier I to Tier IV can appear on a mobs treasure tables (we think). All higher-tier Runes must be extracted from equipment or constructed by combining lesser runes in the Arcane Transmutor. Each table, II through IV, will have one entry that resolves to the next lower table.
This is a Treasure Table. It may be listed in the Drop Table of a specific monster or group of monsters, it may be nested in another Treasure Table, or may list the items for sale by an NPC.