Some operators require an interval to be given specifying the range within which a variable ranges. The sum or product operator are some good examples. They both take as argument the interval giving the range of summation or multiplication. Other operators accepting intervals in some cases are int and condition.
Both in MathML and OpenMath these operators define ranges with
intervals, but differently. OpenMath defines intervals using specific
interval defining symbols found in the interval1 CD. MathML can
use either the interval element or the tags <lowlimit> and
<upperlimit>. These two tags do not have an OpenMath equivalent
and so when encountered must be transformed into an interval. This is
not difficult since one must simply merge the lower and upper limits
into the edges of an interval.