Right before the draft there is a scouting phase. This is where you finish up and get extra knowledge on players you are interesting in.
The amount of points you have to assign during the scouting phase is derived from how much of your staff budget is transferred to the scouting department.
Teams already have some of knowledge about every players. The amount of knowledge depends on two things:
1) how prominent the player is (his draftprojection) and reflects the media hype and exposure a player has gotten.
2) the scouting abilities of your coaches, so if your coaches have high ratings in 'assess ability' you have more prior knowledge about players.
The prior knowledge is manifested in terms of 'free' scouting points that are already on the player, so if you go to a drafteeprofile, you will see a scoutreport with a varying amount of certainty, depending on the two factors above. When you scout a player it then autoassigns scoutpoints equal to the difference between the free points from your prior knowledge and 20. So for instance if you have 12 points for free on a top10 projected player, and click scout it will cost you 8 points. Because the free points are partly driven by media exposure (draftprojection) scouting lowly round 7 projected players is more expensive, but you will have plenty of points to go around.
There is an 'autoscout' button that assigns any unspent points.
Lastly, the coaches also have an 'asses potential' rating and will send you an email suggesting particular players they believe have high potential. Depending on the ratings of the coach the amount and quality of his tips vary.
So: coachratings add to your prior knowledge, scoutdepartment delivers extra points you can assign in the scoutingphase.
Interviews are crucial because they unlock two things. First and foremost, they allow you to see a players personality, which is important in order to get an idea of what he likes, and assess your chances of signing. Secondly, interviewing a player reveals whether he has any special traits (listed under 'special' on his player card). Traits give various bonuses either on or off the field, and thus make the player more valuable. You can only interview a player once, but on the positive side, there is no error margin on the information you get. Scheduling an interview costs 5 scouting points.
Understanding the ratings assesments
When scouting a player, your scout will come back with an assessment of each of his relevant ratings, this means your scout wont be bothered with assessing the tackling of a QB and so on. The most important part to understand about the assesments is that they are based on the average of that particular rating across all active players of the same position. This means that if you are scouting a TE and getting an assesment of his Hands skill, the scout will base his assesment on the average of the Hands of all TE's currently in the league, excluding the draftclass. If the average of all TE's Hands are 63, and the player you are scouting has 63 hands the scout will tell you his hands are average (if he is 100% accurate). Now, because there is a lot of really bad players on really bad teams this means that those averages may be lower than you expect. If you are a top tier team, then 'average' Hands might not cut it for you if you want a pass catching TE.
Margin of error in the assesments
There are 22 (not counting 'unscouted') different assessments ordered by their relation to the benchmark. Observe that the margin of error in your scoutreport will show up in how far removed the given assessment is from the correct assessment in the ordering. This means that as you increase your report certainty the error margin will decrease, and you may be satisfied with less than full certainty when you feel the given assessment likely indicates the approximate level (i.e. the error margin is reduced and you like the indication it is giving).
List of ratings
Remember, these are dynamic and the benchmark will change depending on the playerpool, so not only will it differ between positions, it will also differ from season to season as the playerpool changes. Additionally, because of the number of different ratings, only a few points separate each. In PF 21 these words have been replaced by letter grades F through A.