If your spouse is already in us, she/he may apply for AOS based on following to join. IF outside US, LPRs need to inform a US consulate of their lawful status in order for their family members to apply for immigrant visa. In both following to join cases spouses do not have to wait for visa number so the end result is the same but where the spouse resides affects what procedures he or she needs to follow.
If there is confusion, contact Rijal Law Firm to simplify the process.