Which board to choose for the subfloor?

To begin with, we advise you not to use boards with longitudinal cracks and strong irregularities, as well as twisted ones. What kind of boards will be needed for flooring:

uncalibrated edged;
unedged.

Note that the rough flooring needs further processing, you have to cycle the surface in any case. Smooth bars work better.

When choosing which board to use for the subfloor - also focus on the type of wood. For the arrangement of the lower tier of the sexual structure, expensive solid varieties are not taken, this is impractical. Cheaper softwood is just right for rough work, because it takes the load not directly, but through the finish.

For load balancing, a mechanically strong material is required. Conifers - spruce and pine - are suitable, and larch is very popular in rooms with high humidity.
Also wondering which board to choose for the subfloor - do not chase the most expensive materials. Take grade B or C - they are more budget-friendly and in terms of characteristics are not inferior to analogues.
The main selection criterion is wood purity, focus on the absence of:

fungus;
mold
parasites.

It will not be possible to remove them, we discard the infected material at the purchase stage, carefully inspecting the surface and ends from all sides. To prevent the occurrence of these defects in the future, treat lumber with antiseptics before installation.

The next factor is dryness. Wet sawn timber is contraindicated for a number of reasons:

not tough enough;
quickly covered with cracks;
will be suitable for the growth of mold;
slightly impregnated with flame retardants and antiseptics.

Now about the parameters of such building material as a board for a rough floor. The sizes are standard - 10x600 and 15x600 centimeters, the classic thickness for normal load in a living room on the floor is 2.5-3.5 cm. In case of the expected increased loads or commercial and industrial premises, take the material thicker. Just keep in mind that the total thickness of the rough floor will increase and the height of the room will decrease.

If you are interested in knowing which board is used for the subfloor while minimizing processing, we recommend sheet pile, equipped with tongues and grooves for fixing. It allows you to lay the draft floor gently, with virtually no gaps. Sheet piling of increased thickness is not even necessary to cover with a finish coating, it is enough to sand and varnish.
I hope I managed to exhaustively answer the question. Be careful, check the material before buying and follow the technology of flooring to get a reliable result.