The meaning of "business" is not defined for IHT purposes, so it
has its ordinary meaning which is a trade or profession carried on
for gain.
IHTA84/S103 (3) says that businesses "includes a business
carried on in the exercise of a profession or vocation, but does
not include a business carried on otherwise than for gain". The
effect of this is to exclude "hobby businesses".
In most cases it will be quite clear whether or not a person
was engaged in a business. If there is any doubt you can ask
whether or not the Inspector of Taxes had accepted that the
deceased had been in business. If the point cannot be resolved
reasonably quickly refer the case to your manager for advice.
Similarly where the deceased's description on page 1 of the
IHT400, or something else such as a bequest in a Will, indicates
that the deceased was in business but no business is disclosed
amongst the assets, you should query the position. Do not do this,
however, if it is clear that the deceased had retired from business
or was in fact only an employee.