A Flexi Season allows you to take advantage of a frequent travel discount, giving you 8 separate days of travel in 28 days between two stations, which don’t have to be used consecutively. In comparison, a Season ticket allows you unlimited travel within its validity period - typically for a week, or any period between a month and a year.
Season tickets are available in First Class and Child options, while Flexi comes in standard class only as it is aimed at commuters travelling during peak-times, Monday to Friday, 2-3 days a week.