If it is a subfloor, it isn't intended to be used as the main floor. Based on your picture, I would say this is definitely subfloor (notice that the flooring is not even, and that there appear to be gaps between the boards).

Looks like pine, possibly heart pine.

Thanks— I assume it is subfloor and not meant to be floor because it is nailed right into joists. But is there any reason I couldn’t use it as a floor? The wood looks about an inch thick so I would think it would be strong enough. And I really like the rustic look of it!

Agree, like the rustic look and wide planks. Hope you can use it.

https://www.google.com/amp/s/www.thisoldhouse.com/platform/amp/21016795/how-to-fill-gaps-in-a-wide-plank-wood-floor

Thanks all!

Yep, use it and enjoy. The cheap wood of 100 yrs ago is todays valued vintage timber.

We have some of that in the attic that we're finishing into living space. It's nailed through the tongues and also face-nailed, both into the joists. More gaps than newly installed wood would have, but we decided to embrace it, and finished as-is.

(No, the walls aren't finished yet, and we'll paint the baseboards white)

Weedyacres your floor looks great! The original hardwood on our main floor has more gaps than that! We also have wide plank attic floors, but not as nice as yours. I’ve thought about doing something with them, but the joists are only 2x6, so I figured I’d have to pull them up. Did you have to reinforce the floor joists before finishing the attic?

Our joists are 2x6, but the span is only 10'-6", so we're ok with 40 psf live load, according to the span tables. What's interesting with the existing floor, however, is that there's a bit of a bow in the floor (like every old house), but it's across the joists, not along them. So it's not a joist sag.

We are being conservative with weight, however, We'll not put stone tops on the vanities, I'm using Kerdiboard instead of cement board in the shower, etc.

What WAS tricky with 2x6s was the clearance for drains. Not enough for toilet and shower drains. So on the toilet, I traced around the base and made a pedestal of sorts (you can see it in the upper RH corner of the photo above). For the shower, we needed to use self-leveling compound for the base. We put the drain on the low side, and built up the SLC about 1" higher than we would have needed to. Presto: drains with proper slope!