Numbers aren't just numbers in ISO8601 when considering the extension formats. In some situations they may:

• Have exponents
• Have significant digits (the S suffix)
• Have unspecified digits

And how these are formed varies by whether the number is being parsed for an implicit form, extended form or explicit form.

Implicit form

• Numbers are always positive with no sign except when its the year in which case it may have a negative sign.

• Numbers are always a positive integer, or a the "unspecified" symbol X in any digit location. Numbers are either 2, 3 or 4 digits wide (decades are three digits) and this implementation does not currently support more than 4 digits for years.

• Significant digits are not supported.

** Extended form

• Same as the Implicit Form

Explicit form

• Numbers may be positive of negative

• The "unspecified" symbol X may appear in any digit location.

• The symbol X* means the entire field is unspecified.

• Exponent and significant digits are supported, but only if the number is an integer (ie does not have unspecified digits).