Post Reach vs Post Engagement
After further research, Post Reach is how many people you are able to reach on Facebook with your advertisement ads. I think the goal is to have as much exposure as possible. The ads will reach other fans and non fans through their friend's news feeds.
Post Engagement is the reaction of the receivers or audience to your advertisement ad. They most likely will select the like option and or comment on your ad but may not follow through with a purchase. It is possible to not receive post engagements on an advertisement but still receive a lot of purchase. That most likely will happen when an ad comes across a previous customer that was very satisfy with the product.
For GiantSkillz post reach and post engagement has been very positive for us. One of the main reasons is because of the final product that we produce and that is a lot of these trainees are able to land Division 1 scholarships with major universities. Since we are one of the very few training organization for Defensive and Offensive Linemen, we get a lot of attention because other training organizations focuses of skill position players such as Wide Receivers, Running Backs, Defensive Backs and Quarterbacks.