select-option: is used for the whenever create the
select-options the system creaat the one internal table same
variable name. that table name is called selection table.
the main purpose of the internal table is used to stored the
selection text.but at the same time the system create the
Sign ,Option,Low,High implicitly.
it will accept the value ranges from the selection screen.
means lower range and higher range value.

parameters: is used for the it will accept the value from
the user.the system doesnot create the internal table
implicitly. it will create the explicitly.

For parameters if we use value check, the value given in 
taht field will be validated aggenest the check table 
automatically, for select options it is not posible.