Vacuum level is perhaps the most critical consideration as it affects each of the other factors.
Typical classifications and ranges include:
Rough vacuum: from from 10 3 mbar to 1 mbar
Medium vacuum: 1 mbar to 10 -3 mbar
High vacuum: 10 -3 mbar to 10 -7 mbar
Ultra-high vacuum: 10 -7 mbar to 10 -12 mbar
Extreme high vacuum: less than 10 -12 mbar
Different pump technologies are required to achieve different vacuum pressure ranges. Depending on the target vacuum level, a mix of different vacuum pump technologies might be necessary.
Forevacuum pumps — those operating in the rough and medium vacuum ranges — exhaust to the atmosphere and can operate in isolation.
High and ultra-high vacuum pumps — such as turbopumps and diffusion pumps — need to exhaust to, or operate with, a forevacuum pump.
Ion, non-evaporable getter pumps, and cryopumps need initial evacuation and occasional forevacuum and high vacuum pump support at certain stages.
Related: Brush up on the unique demands of working with high vacuum ranges on this resource page by Vacuum Science World, The Fundamentals of High, Ultra-High & Extreme High Vacuum. Or, check out this guide by Vacuum Science World, to Understand the Different Types of Vacuum Pumps in more detail.