Is "Substance" The Current Buzzword?

It seems that one of the latest buzzwords in the field of international tax planning is "substance". The term "substance" is frequently used to indicate real/physical presence of legal entities in their country of tax residence, as opposed to the use of the so called "shell" or "conduit" companies for obtaining certain tax, or double tax treaty, benefits.

But what does "substance" really entail?

Is it enough to merely rent or have available some sort of office space (sometimes shared with other entities), or is the employment of personnel also required? And when we are considering the employment of personnel, is the existence of secretarial or administrative staff sufficient, or does higher ranking managerial staff need to be employed as well?