ISO 4217 defines 3-letter currency symbols:
Do currencies\' minor units (cent, pence
You need to look at the standard itself.
From the ISO website:
ISO 4217:2008 specifies the structure for a three-letter alphabetic code and an equivalent three-digit numeric code for the representation of currencies and funds. For those currencies having minor units, it also shows the decimal relationship between such units and the currency itself.
ISO 4217:2008 also establishes procedures for a Maintenance Agency, and specifies the method of application for codes.
The key bit is:
it also shows the decimal relationship between such units and the currency itself.
So to answer your question, I couldn't find an ISO Standard that discusses minor units. Similar standards discuss Commercial Administration and Finance.