@Steven has it correct that it's most likely SPF for generic dimensional lumber.
In the central US, you can also find dimensional lumber in Douglas Fir (DF) and Southern Yellow Pine (SYP). Usually SYP is used for pressure-treated lumber, but you can find it non-treated as well. Both are a higher-grade material than SPF and are generally used for members that resist bending (joists and rafters, for example).
Since DF and SYP carry a premium over SPF, they're usually labelled pretty clearly.
Not that this will apply for your case, but on the west coast you can also find redwood in dimensional lumber. I haven't seen too much of it east of the Rockies, but hopefully someone over on the west coast finds this useful.