Probably the most obvious example is the expanded representation of a year. 8601 says that a year can be more than four digits, but if so the number of digits must be agreed upon in advance by communicating parties -- i.e. by private agreement, outside of the standard, and the means of communicating such agreement is not provided. You may recall, we (that is, this forum) completely rejected that approach as incompatible with the concept of interoperability, and came up with a separate (and interoperable) solution to represent a year requiring more than four digits. In fact I think this is the only case where we rejected the 8601 approach.
> -----Original Message-----
> From: Discussion of the Developing Date/Time Standards
> [mailto:[log in to unmask]] On Behalf Of GERRY ASHTON
> Sent: Thursday, August 17, 2017 5:04 PM
> To: [log in to unmask]
> Subject: Re: [DATETIME] Combining time zone with day precision
> > On August 17, 2017 at 8:54 AM Christoph Päper
> <[log in to unmask]> wrote:
> > but it is not incompatible with any format, so it would be a valid extension if
> the partners in information exchange agreed upon it.
> The ISO 8601 standard has a number of cases which are explicitly mentioned
> may be done, but only upon agreement of the data exchange partners. I've
> always interpreted this as meaning that extensions that are not mentioned at
> all are not acceptable, even if the extension doesn't contradict any part of the
> standard. Certainly parties who made such an extension would be unprotected
> against future updates to the standard that turn out to be incompatible with the
> parties' extension.