If you are inside IR35, your ‘fee payer’ is the party above your Limited Company (PSC) who is responsible for deducting national insurance and tax before paying you net earnings.
For most contractors, the fee payer is normally a recruitment agency, since they are the party above the PSC responsible for transferring funds into your account. However, if your PSC is paid directly by the end hirer then they will be the fee payer, which means they would be responsible for deducting tax and National Insurance contributions from your net invoice.