This may seem trival... But what does AFV mean?

Also USA tanks have designations lke M5A1 or M4A3, what do these mean?

AFV stands for Armored Fighting Vehicle.

The naming convention of the US military is interesting. The M means its non-exeperimental and is in production. The number is a sequential number for the given system. The extended lettering/numbering all designate different upgrades to the system.

Here are some links to this type of discussion
https://armorama.kitmaker.net/forums/7707&page=1

And this one too.
https://armorama.kitmaker.net/forums/4183&page=1

The "M" basically stands for "model". The M is usually followed by a number and together they become the nomenclature. There is usually verbage that goes along with the nomenclature, like Tank, Medium, M3 or Tank, Light, M3. This is why there used to be many M3 vehicles (half-track, Lee/Grant tank, Stuart tank). They have tried to avoid this lately, but we still get several M9s (M9 ACE, M9 9mm pistol, M9 tank dozer blade).

In Canada it's a C. C7 rifle, C6 GPMG, Leopard C1 and C2 etc. :-)

I believe a similar system is used by urinal manufacturers to indicate the number of punters that can be accomodated, ie, P1, P2, P10, etc.

Regards Dave