arithmetic evaluation question

[dino bartolome]

this is a trival bash question but i'd like to know the meaning of this line
echo "${ac_cv_func_finite+set}"
if the ac_cv_func_finite shell variable is set the output is "set" but if it
is not initialized the output is nothing. What is the meaning of the +
operator? According to the manual:
The value of a parameter is coerced to a long integer within an expression.
and where can I find this information in the gnu bash reference?

many thanks in advance for any help.