What are considered as 'Eligible Purchases' to Amex and credit card companies (Spending Minimum to Get 100K Points etc)

I need to spend 1500 to get my 100K points within the first 3 months. Won't have any trouble spending the 1500, but what exactly are 'eligible purchases'. Does it basically just mean spend it at a place that isn't dodgy, or not properly registered as a business or Amex facilitator etc..

Is there anything legal that they won't let you spend the minimum on?

Thanks guys