PSL stands for Pancreatic Sensitive Lipase.
Pancreatitis is a challenging diagnosis to make in the cat, as there is no single good test. Some tests will have false positives, others false negatives.
Whether a sample is fasted vs non-fasted seems to make a big difference for any lipase tests. Most of the time, cats are not fasted before their blood work, which can make interpretation of results more challenging.
The laboratory (presumably Antech for PSL) sets the reference range based on a certain percentile (e.g. 99% of "normal" cats fall between 8 and 26 U/L). The problem is that a fraction of normal cats will fall outside the reference range, and the ranges are based on an arbitrary population that is not necessarily representative of your particular cat's population.
As to what Antech means by a mild or moderately high elevation is difficult to say, and I cannot find any good documentation for the test. With this particular test, I would become more concerned for pancreatitis at roughly 2-3 times the upper reference limit (so around 50-75 U/L). I have seen values for this test at 200+, which is obviously more diagnostic. That being said, the diagnosis of pancreatitis is multi-factorial. Are any clinical signs present (vomiting, inappetance, abdominal discomfort)? Is there evidence of pancreatitis on imaging (radiographs or abdominal ultrasound)?
PSL and fPLI can be elevated due to inflammation in the region of the pancreas (e.g. duodenal or other intestinal inflammation). Therefore it can be useful to pursue a GI panel (folate and cobalamin levels) as well as further imaging to determine if there are pancreatic changes or visible intestinal thickening suggestive of inflammatory bowel disease or GI lymphoma (common in older cats).
I am more familiar with Idexx's fPLI, as I run that more routinely. Commonly I see mild elevations in otherwise clinically healthy cats, and in those cases do not jump to further testing or treatment immediately.
I try not to run blood tests for pancreatitis unless there is a clinical suspicion for pancreatitis, because the results can be so subjective. With pancreatitis, it is important the vet treats the cat not the number.