What is the difference between Sotalol and Sotalol AF?

Sotalol AF is specifically used for maintenance of normal sinus rhythm [delay in time to recurrence of atrial fibrillation/atrial flutter (AFIB/AFL)] in patients with symptomatic AFIB/AFL who are currently in sinus rhythm.
Plain Sotalol is not approved for AFIB/AFL indication and should not be substituted for Sotalol AF because only Sotalol AF is distributed with a patient package insert that is appropriate for patients with AFIB/AFL.

[Extracted from https://www.drugs.com/pro/betapace.html
Sotalol hydrochloride (Betapace®) is used to minimize the risk of induced arrhythmia, patients initiated or reinitiated on Betapace® should be placed for a minimum of three days (on their maintenance dose) in a facility that can provide cardiac resuscitation and continuous electrocardiographic monitoring. Creatinine clearance should be calculated prior to dosing.

Sotalol is also indicated for the maintenance of normal sinus rhythm [delay in time to recurrence of atrial fibrillation/atrial flutter (AFIB/AFL)] in patients with symptomatic AFIB/AFL who are currently in sinus rhythm and is marketed under the brand name Betapace AF™. Betapace is not approved for the AFIB/AFL indication and should not be substituted for Betapace AF because only Betapace AF is distributed with a patient package insert that is appropriate for patients with AFIB/AFL.]