If we look at R&D tax relief schemes, SME R&D tax relief is classified as notified state aid, whereas research and development expenditure credit RDEC is not.
So, in accordance with EC rules if a company has already received notified state aid then its access to SME R&D tax relief will be ceased. Due to this limit even if you fall under the category of SME, you can still claim R&D Tax Relief via the RDEC scheme.
RDEC scheme is less generous than SME, as it only grants 13% relief on qualifying expenditure rather than 33% like SME R&D tax relief scheme.