Hey @sgrodpm,
Apologies as I've been focused on a project at work and missed your message.
Here are the main criteria for any good underlayment:
Minimum 1/4" (6mm) thickness - many of the products out there are 5.5mm so make sure it's stated that it is 1/4"(6mm), not a nominal 1/4"(6mm).
Must be sanded, filled (if needed) and free of voids inside
Must be put together with an exterior grade adhesive (not pressure treated or treated - most APA standard plywood panels are put together with an exterior grade adhesive.
Must not contain dyes that can leach or bleed
**Most important in my opinion** must have a written warranty from the manufacturer for use as an underlayment panel. If the panel has this, it was designed for use as a flooring underlayment.
I believe APA A-C rated plywood typically meets all of the requirements. However, it can be difficult to find one that has a written warranty for underlayment. If you do find one that has a warranty, say from someone like Weyerhauser, those are typically decent products and would be usable for your application..
As a side note, for many years birch 4'x4' or 4'x5' underlayment was considered the "gold standard" for underlayment. Unfortunately, most birch used to make plywood has been in short supply since the war in the Ukraine began. If you can find birch plywood, it's going to probably be crazy expensive, but it is a incredibly good product.
That being said,
Here are two products that I'm personally familiar with that meet the requirements and are well made...
Matrixx from Traxx industries:
https://www.traxxcorp.com/products/matrixx-underlayment/
Ironply from Patriot Timber:
https://www.patriottimber.com/ironply/
I would feel safe using either of these in my own home with regards to their manufacturing and performance.
Hope that helps and all the best in your project!
Here's a Monarch from one of my recent walks for good luck!