It is a sales jargon term that has no meaning. Below is an answer I gave earlier:
I have never gotten a definition of “high-tension/low-tension” that makes any sense to me. So I am suspicious whenever anyone talks in those terms.
In general, the longer the piano, the longer the strings. It is a physical law that the longer the strings, the higher the tension the strings need to be to give the same pitch. Also, the heavier or thicker the strings, the higher the tension needs to be to give the same pitch. These are physical laws, regardless of the make of piano.
To the best of my knowledge, every piano manufacturer has used heavier strings the longer the piano. Even without that, no matter who made the piano, bigger pianos would have higher tensions than smaller pianos, just from the length of the strings.
There are a couple of rules that should be followed. The most obvious is that you do not want the tension near the breaking strength of the string. In fact, you want it under the elastic limit of the string, which is about half the breaking strength. This is usually not a problem, except that the breaking strength will usually drop considerably when you go from the plain wire strings to the over wound strings.
The other rule is that the tension should not change a lot from note to note. Everything else being equal, lower tension will not be as loud, and it can be boomer than higher tension. This may not be noticeable if you have gentle transitions from one area of the piano to the next, but big jumps and rapid transitions can be heard.
Unfortunately, when calculations were more laborious than with today’s computers, when most manufacturers did not see the value of designers that were as adept with the slip stick or log table as they were with the French curve, there were some seat of the pants practices that were pretty much ingrained in piano design which were not ideal, and in some cases, pretty bad. These include changing the wire size too often at the top of the piano, and not changing it enough near the transition to over wound strings. The result is a scale which is way too high in tension just below the top octave, and drops way too low in tension way too quickly near the transition.
Careful redesign should give tensions that avoid too high tensions, and in shorter pianos, drops gently to near the tension of the over wound strings, or rise equally as gently in longer pianos, and avoids any big jumps anywhere in the scale.