What is this about?

Only guessing a few of the buttons don't work  ask the seller directly

sticker was orig from MTH and on the box. it was printed onto the box as part of the packaging

I have never seen such a sticker but it might be indicating that the "wave" produced is not a pure sine wave and may cause some trains (even early MTH ps1) difficulty. This was a serious issue briefly in the mid 1990's and led to the marketing of "pure sine" transformers. I have an early MTH ps1 northern that refuses to leave neutral when used with my z-750 but this is the exception, not the norm.

Uberstationmeister and Railking are on the right track.

QSI reverse units and subsequent sound systems were designed for Lionel postwar transformers. Around 1990, when new electronic transformers came along, QSI units could "read" their waveforms incorrectly and malfunction. This is the first time I have heard of a sticker. It probably addressed that situation, as RailKing posted. I think MTH corrected the problem in later production runs of transformers. The crowning achievement was the Z4000 with pure sine wave and electronic programming of RESET Features in QS-1.

Back in October, about a week before the  York Meet, I purchased MTH Premier Southern Ps-4 No. 1401 (20-3006-1) from a Forumite. Made in 1994, she was among the first to have QSI "ProtoSounds." This elegant Georgian locomotive ran with my postwar ZW but not with my MRC Pure Power. I had a hunch that might happen. MTH Parts brought a board and a chip from a Railking Ps-4 (30-1125-1) made in 1997. Wayne Renga of J & W Electronics installed them. Now my Ps-4 runs with other transformers, and I gained Passenger Station Announcements (PSA).

I took a photo of the original circuit boards in the tender. I'll post it here. The only programmable feature is (6), Chuff Volume.

I came across this warning in a QS-3000 user guide (1999): "Early Z-750's packed in yellow boxes produce damaging voltage spikes and overvoltages, and are not recom-mended. Later Z-750's packed in red boxes deliver a lower voltage and are safe to use. To determine which Z-750 you have, attach a PowerGuard [discussed in a recent topic] to the transformer and turn up the power. If the red light on PowerGuard stays on steadily, you have the early Z-750 that is not recommended. Damage to QSI sound systems and reverse units from these early transformers is not covered under any QSI warranty."