Acquiring a UK citizenship determines whether you will receive a naturalisation certificate or a registration certificate.
You will be issued a British certificate of naturalisation if you receive British citizenship through naturalisation.
Similarly, you will be issued a registration certificate if you gain British citizenship through registration.
Previously, certificates of registration were given to nationals within the European Union, Switzerland, Iceland, Norway, as well as Liechtenstein (EEA). 
However, these registration certificates are not valid anymore. Therefore, the EEA nationals and EU citizens should apply for another form of permission to be allowed residence in the United Kingdom.
A good example is a settled or pre-settled status via EU settlement Scheme (i.e. EUSS). This is only applicable to individuals residing in Great Britain before 2020.
If you apply to EUSS, you must have reasonable grounds to apply (for example, if you were sick or a survivor of domestic violence).