Capacity to work less than 15 hours

So my dsp claim was accepted the other day, I got a letter in my inbox saying a partial capacity to work of less than 15 hours and that I need to do a job plan which they will call me about but also that I can volunteer to meet my requirements. The wording makes it sound like I both have to do a job plan and also that I can opt out? Not sure how to take this any advice