I'd guess "standard" 1 ohm coil for example may have DCR of .7 or .8 while true would be 1.0. Your amp should "see" whatever true number is published. It is highly likely if you need to worry about this to optimize some sort of top tier SPL build just ordering blind won't help anyway as your actual impedance in your box in your car will be a complete unknown until the sub(s) are installed and tested.
Long story short, I'd buy "standard".