This Study Mind Tutorial covers the following specification points:

Edexcel: 1.6-1.7

1.6. be able to define the terms ‘relative isotopic mass’ and ‘relative atomic mass’, based on the ¹²C scale

1.7. understand the terms ‘relative molecular mass’ and ‘relative formula mass’, including calculating these values from relative atomic masses Definitions of these terms will not be expected. The term ‘relative formula mass’ should be used for compounds with giant structures.