Interior slabs and footings that stay out of the weather
The number by itself does not say much until you know where it is headed. A 3000 PSI mix is the kind of spec that shows up on a basement floor under a finished house, a footing that will sit below grade under a heated slab, or interior work that will never see a Virginia winter directly. Nothing about that concrete is undersized for what it is asked to carry. It is simply built to a lower bar because the exposure it faces is lower.
That is different from exterior flatwork in this part of the state. For most of what gets poured outside here, we spec a 4,000 PSI air-entrained mix, because that concrete goes through freeze and thaw every winter and a plain 3,000 PSI mix without entrained air is not built for that cycle. The strength number and the exposure it is ordered against are two separate questions, and a mix design answers both at once.
The strength spec belongs on the order, same as everything else
Whoever books the concrete tells the plant what strength is going on the ticket, and the plant proportions cement, sand and stone to hit it. On our own work that conversation happens before anything is ordered, not after a truck shows up. If the load is also going through our line pump, the strength spec and the pump spec both belong on that same order, since they are two separate pieces of information the plant needs, not one.
What strength your particular job actually calls for is not something to guess off a glossary page. A slab under a roof and a slab open to the sky are not the same order, even if they look similar on a plan. The same is true of two footings on the same property, one buried under a heated addition and one holding up an open porch.