Reverse engineering the acceptance rate? Assume that we have an educational instance X which requires a certain prerequisites (mininum TOEFL, GRE score, etc.). In total 100 applications have been received and 50 students have failed to score above the minimum threshold while the remaining 50 have scored above it. The acceptance rate is given to be 10%.

Is it calculated basing on all applications or only the applications that have passed some certain criteria?

In this case, 


if taking into account all applications => 10 students accepted
if taking into account only those who meet criteria => 5 students accepted
I would suspect that the acceptance rate is calculated against the total number of applicants (so 10 acceptances). But, unless you read the number in some official document that also precisely specifies how exactly this number is to be computed, I don't think you can (i) say for sure what it means, (ii) assume that the number is accurately computed against any particular approach you know, or the person who computed the number knows. For example, if you get the percentages of acceptances from a department web site for the previous 10 years, you have to expect that these ten numbers will have been computed by different people and with different definitions.