What's the selection criteria for the kingdom 'protista'?

Question

If possible do you also know any advantages or disadvantages of this system.

Answer 1

The kingdom Protista is unique among the six classification kingdoms. If one were to look for a common bond linking all the organisms of this kingdom together it would be difficult to do so. Most of the organisms are unicellular, though many group together forming colonies. Some are autotrophic, while others are heterotrophic. The main reason these organisms are grouped into Protista is that they cannot be satisfactorily placed in any of the other kingdoms. Protists form two groups: heterotrophic protists and autotrophic protists.