That is a list of the characters you want to keep from the email adress when comparing to the original content.
keepchar('bonjour','jour') = 'bon'
keepchar('Abcd123','123') = '123'
purgechar is the opposite, it define which characters should be removed :
purgechar('bonjour','jour') = 'jour'
purgechar('Abcd123','123') = 'Abcd'
Have a look in the help files. They have often good examples. As a beginner though it is dificult to know "what" to search for. But if you search for "string functions" in the help, you will find a lot of useful functions.
Oh, ok. Yes I added those because they are valid email characters and might exist, so they are needed for the comparison, in a general pupose. Perhaps they are not valid in your organization, if this is the case, just remove them.