An Earnings Factor (EF) is the amount of earnings in a tax year on which
Class 1 or Special Class 2 Contributions have been paid and credited.
29. Each year an amount is set to allow for the different ways in which
employers calculate the contributions due on both weekly and monthly
earnings. This is called the Make Up Provision.
30. For this to apply, the EF must partly or wholly come from Class 1
contributions.
31. The EF is calculated by NPS and shown on the eNIRS2 browser, or in box
5 of form RD26:

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Class 1 Contributions
18. Class 1 Contributions are based on a percentage of earnings between the
Lower Earnings Limit and Upper Earnings Limit.

Class 2 Contributions
21. Self-employed people pay Class 2 Contributions at a flat rate. Share
Fishermen and Volunteer Development Workers pay Special Class 2
Contributions

https://www.whatdotheyknow.com/request/163678/response/40476...

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Note added at 11 hrs (2019-03-11 21:58:48 GMT)
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Credits: post-1975 credited earnings (credits): overview
From 6 April 1975, in line with the move away from a flat rate contribution to a contribution liability based on earnings, Class 1 credits were awarded at a standard rate equivalent to a payment of weekly earnings at the lower earnings limit in force at the time. From 6 April 1987, instead of a Class 1 credit being awarded, a person is instead credited with an amount of notional earnings equivalent to the Lower Earnings Limit (LEL) in force at the end of each pay week (Saturday to Sunday). However, “credited earnings” are normally referred to as “credits”, although this is not the term used in legislation so may not be appropriate in a formal legal document. Class 3 credits continue to be credited contributions.

The customer’s NI record shows an amount of “earnings” for the period for which credits are due, and the number of “credit”.
https://www.gov.uk/hmrc-internal-manuals/national-insurance-...