The higher the STL level the lower the number of spurious trips caused by the safety system.
A safety function that activates without the presence of a dangerous situation (e.g., due to an internal failure) causes economic loss.
The STL is a metric that is used to specify the performance level of a safety function in terms of the spurious trips it potentially causes.
The spurious trip level represents asset loss due to an internal failure of the safety function.
This actually depends on many different factors including the financial strength of the company, the insurance policy they have, the cost of process shutdown and startup, and so on.
Standards do not define STL levels because they do in first instance not represent safety but economic loss.
The SIL level expresses the probability that the safety function does not work upon demand from the process.