Here are the definitions, not for products but for human competence, according to the DOD.
What is Suitability? Suitability refers to a person’s identifiable
character traits and conduct sufficient to decide whether employment
or continued employment would or would not protect the integrity or
promote the efficiency of the service.
What is Fitness? Fitness refers to the level of character and
conduct determined necessary for an individual to perform work for,
or on behalf of, a Federal agency as an employee in the excepted
service (other than in a position subject to suitability) or as a
If I understand well, and adapted for a product, suitability, would be, for example, that a knife is suitable to cut a steak (whatever the state of its blade) but could not be called fit to cut the steak if it's not sharp enough to do the job.
See also this reference about commercial transactions and fitness vs suitability in the Australian legal system: sydney.edu.au/law/slr/slr_32/slr32_2/Pearson.pdf.
It's might be confusing though. The author seems to say that suitabilty is the continuation, a more modern verson of fitness but in the body of the text he sometimes uses the term as synonyms...