The program sees the commas as the value dividers.  Thus, you cannot use commas in the combo box list of values.

If you have a list of:
ABC
DEF
GHI
you would enter this as ABC,DEF,GHI

Ex1.  If you want to have JK,L as a value, (this will not work)
then when you enter:
ABC,DEF,GHI,JK,L  then JK and L are seen as two different values.

Ex2 , If you enter ABC,DEF,GHI,"JK,L" you get:
ABC
DEF
GHI
"JK
L"
as values.