A string parameter can be as long as 255 characters.
When concatenating two strings (with a Calc block expression, for example) whose combined length is less than 255 characters, the resultant string is the first and second string combined. For example:
StringA = "1234567890"
StringB = "abcdefghijklmnopqrstuvwxyz"
StringA + StringB = "1234567890abcdefghijklmnopqrstuvwxyz"
When concatenating two strings whose combined length is greater than 255 characters, the resultant string is the first string appended with three underscores (___). For example:
StringC = "1234567890"
StringD = "abcdefghijklmnopqrstuvwxyzabc...qrstu" (255 characters)
StringC + StringD = "1234567890___"