How do u identify long-lead equipment in your schedule?

I have 3 questions for the well experienced planners/schedulers who have developed integrated EPCM schedules:

1. What is the concept of a long-lead equipment? Is it determined by size, cost or other characteristic?

2. Which equipments are considered as long-leads for oil/gas, mining and power plants projects?

3. How do you identify long-lead and schedule-sensitive materials and equipment in order to put it in your schedule?

Thank you.