[Tagging] maxweight : x tons exept for bus

[Marc_marc]

Hello,

a hithgway have a traffic sign "maxweight x tons exept for bus"
how to tag it ?
can you choose best practice and tolerable values that should
be documented?

1) maxweight=x + maxweight:bus=no
it follow the classif yes/no as basic value but it not numeric

1bis) maxweight=x + maxweight:bus=none/default/unrestricted/no_limit/...
many alias for the same thing

2) maxweight=x + maxweight:bus=0 and disallow textual value for maxweight*
this solve the "hidden rule" that some mapper have about numeric-only 
value for maxweight*

3) maxweight=x + maxweight:bus=none @ bus and disallow textual value
for maxweight*
this solve the "hidden rule" that some mapper have about numeric-only 
value for maxweight*
same as 2 but longer/more complete (and I don't see the added value of 
it for this basic sign)

4) other idea ?

Regards,
Marc