web CSS UNITS

rem = responive to setting at html main body (16px as web standart)
em  = responsive to Global Body Settings Theme Builder (14px standart Divi PostContent)

if use em/rem for objects/elements/containers as a unit always response to the font setting:
-NOTE: if use REM: alwas respons to Root Setting (16px by default)
-NOTE: if use EM: always refer to its outher body/element/section/paragraf/container setting
–>thats why the em is refering to the Divi ThemeBuilder Post Content

Only Use REM for Borders, Padding, Margin, any setting u want responsive to fontsize for automatic size(zoom) at change of browser settings.

1rem = 16px

1em = 14px