The payment frequency tab sets how often the payment will occur, in most cases it will be set to "Once off"
Once off - the payment only occurs once ( most common ).
Series - a second or third payment can be set at a later date on the "Event Prices" tab. (Must have at least 1 Event Category)
Recurring - the payment will occur at an ongoing set period, eg Monthly.
Once an event is setup with Event Categories and pricing details and thus "Finished", the above 3 options are greyed out and cannot be changed.
Register Now / Registration Logic can change it for you, however we will also advise what other changes need to be adjusted with the event setup when the changes are made.
Series - Charge the person $100 today to register, then $50 in 1 months time, then $50 in 2 months time - thus a total payment value of $200 spread over 3 months.
Recurring - Charge the person $40 today, and then it will charge them $40 every month on the same day each month until the credit card expires.