There is information on the SF-50 codes for block 27 here: https://www.opm.gov/retirement-services/calculators/fegli-calculator/codes-on-a-notification-of-personnel-action-sf50/
Another helpful resource is
Guide to Processing Personnel Actions
Dear Ms. Robinson,
Thank you for posting your request on History Hub!
There are code lists on the second page of the OPM version of the SF-50 .pdf. You may also consult NASA's 'Understanding the Notification of Personnel Action Form SF-50', which goes into detail of all of the fields in the form.
We hope this information is helpful.