ETASSUM42150 | Schedule 4 Company Share Option Plan (CSOP): Eligibility of individuals to participate: The employment requirement: Secondment of employees
From HM Revenue & Customs · Employee Tax Advantaged Share Scheme User Manual
An individual employed by one company may be seconded to work full-time for another organisation, but continue to have his or her total remuneration paid by the first company (which invoices the other organisation for the individual’s services). The individual may be accepted as still employed by the first company for the purposes of eligibility to participate in its Schedule 4 CSOP scheme if:
the individual’s service contract is solely with the scheme organiser (or other constituent subsidiary), and
any contract to which the other organisation is party is with the “employing” company, not with the individual.