I presume that the assessment fee is to establish that you're at the appropriate level of knowledge and experience for that level of membership. Which is why it's a one off amount. Then the membership fee is the annual cost of maintaining that level of membership. You could always ring the Membership helpline for a definitive answer.

Hi Kyla
I'm an Assessor within the Assessment Team. The difference in fees is because the 'One-off assessment fee' is the fee charged for your full experience assessment (EA). As you've probably already read on the EA webpages, this is our alternative route in to membership if you don't want to study for a qualification and already have experience as a people practitioner. The membership fee is charged to bring you into membership (giving you access to a whole host of fantastic information, reports, research, data and other support materials).
Hope this helps.
Olivia

As the article is titled how much does Experience Assessment Cost then like Jackie I believe the "one off assessment fee" is the fee to carry out the assessment of your experience if you aren't qualifies to join the institute by allied course / examination etc.

Thank you all - I understand now!