The topic you choose must have enough depth that you can create a lot of content for it. This is important for building an authoritative site, for search engine optimization, and most importantly, for the end user. If you don't have enough content about a topic, you're not going to be taken very seriously as an authority on the topic and it's unlikely you can convince someone to make a purchase from you.
The terms of an affiliate marketing program are set by the company wanting to advertise. Early on, companies were largely paying cost per click (traffic) or cost per mile (impressions) on banner advertisements. As the technology evolved, the focus turned to commissions on actual sales or qualified leads. The early affiliate marketing programs were vulnerable to fraud because clicks could be generated by software, as could impressions.
Medium is an online community where anyone can post articles and essays and get them in front of their audience of millions of monthly readers. While it can be a great place to build your audience for your own blog or find customers for your consulting business, with Medium’s new Partner Program, you can now make money online whenever someone reads your articles. Register for free and you can choose whether your articles are freely available or only readable by those people paying $5/month for a premium Medium membership. You’ll make money based on the amount of people who read and engage with your posts each month.