control characters on track 3 in magnetic stripe cards

["S.S." <shanksauce@xxxxxxxxxxx>]

Values 0xA, 0xC, 0xE (aka ":" "<" ">")are noted as "Control characters"
in ABA/ISO 7811 spec for track three data. What does this actually
mean? Does anyone have any examples of how they are used? I understand
how stop, start and separator are used on a magstripe card but not
control characters.

I would like to use the values 0xA, C, and E, as well as 0xD (separator
value - "=") as meaningful digits in a proprietary encoding scheme (in
order to cram more application usable data on the track). Are there any
possible drawbacks to this? For instance, it's not a good idea to use
0xF (stop sentinel - "?") in such a manner as I've seen encoders which
(correctly so) would interpret this as the end of your data and not
encode any digits sent after that. Likewise the start sentinel; so my
scheme will have the start and stop sentinels in the right place.
However, it seems harmless to scatter these other 4 characters
throughout my stripe in ways that are meaningful to my application. But
before I commit to that, I would like to make sure there are no
problems with using these values in such a manner. Examples of problems
would be that certain encoders wouldn't play well with this usage, or
certain card encoding software packages would not play well with this
usage.

Thanks!

Dave