Season numbering is defined by Extended Date/Time Format (EDTF) specification (https://www.loc.gov/standards/datetime/) which became part of ISO 8601 in 2019. See '§Sub-year groupings'. The standard defines various divisions using numbers 21-41. cs1|2 only supports generic seasons. EDTF does support the distinction between north and south hemisphere seasons but cs1|2 has no way to make that distinction.