Hi Mick,
Custom fields of certain types consume multiple custom fields from your organization's quota:
The data types and their allotted value are as such:
Duration fields count as 3 fields
Reference to Object fields count as 1-3 fields depending on the relation selected (Selecting a super class such as Work Items or All Issue Types, uses up 3 fields, while using a leaf class, such as Tasks or Risks, only counts as 1 field)
Currency fields count as 3 fields
Regards,
Tamir