Just take a rummage inside the Avast Settings, especially the Custom Scans section.
There are really a lot of options more than can easily be explained in the forum.
See Image example.
I think the custom Full Virus scan basically recreates the preset of the Full Virus scan. You can customize it, but if you don’t then it’s just the same as the preset.
‘Virus Scan->Scan Now->Full Virus Scan’=Runs a manual full scan immediately (but takes a longer amount of time).
Custom Full Virus Scan= Runs a manual scan immediately (and takes a shorter amount of time) and, further, the full scan can be customized with specific parameters so that only predefined areas are scanned. This option would be chosen if a ‘Full Scan’ is unnecessary.
*Incidentally, David, ‘obvious’ is subjective to the individual who already ‘knows’ the specified information.
Essentially, all I wanted to know is if a Manual Full Virus Scan scans ‘everything’ (without the ability to set any parameters) whereas a Custom Scan Full Virus Scan has built-in parameters so that only pre-selected areas are scanned.