The notches located on the top and bottom of the carriage bars (or sideshifting attachment) allow the safety lock pin assembly to prevent unwanted lateral movement of the forks.The ITSDF B56.11.4-2005 lays out ranges for the notches, but each manufacturer has they're own pattern.
According to a Cascade representative, they approach their sideshifter attachments to mimic the lift truck manufacturer's specifications. For a generic approach they place a center notch followed by notches that will allow forks to rest together in the center, based on standard fork widths for the capacity unit involved, then place notches 3" on center out to and including spacing that allows maximum fork spread available.
For accessories that have fork pockets, it's advisable to check the available fork spacing.