Sentinel values, or, Transversal Path terminators, in programming, are used as conditions of termination, usually in a loop or repeating algorithm. They do not hold or reference any data managed by the data structure, and thus, computation doesn’t encounter it as a legitimate processing output. In other words, a sentinel value is chosen typically as data values that the loop won’t meet, and hence not perform with.

Sentinel values can be used whence the number of repetitions required for a 'looping process' are unknown before the actual repetition structure begins, and also to signal the end of user input. Hence why algorithms which repeat until a sentinel value is seen are known as sentinel loops.They are also typically associated to file-streams.

Can be used in programs where test scores are provided, and when a certain value is inputted, it may result in termination of the loop, and lead to the average being printed.

In that example, -1 as the s.v. is used as it is an algorithm which computes non-negative integers, and so such a value can be set as the sentinel value as it will never be encountered as a legitimate processing output, and this is basically that third example right there.