Token diagnosis - why would a custom SFDC field not show up as an available token?

[Anonymous]

We have a custom field on our lead records that for some reason is not showing up in the list of possible token values. Are there any specific requirements for a custom field to be available for us as Marketo token?

I did notice that a similar custom fields is available as a token, with the only difference I see being that this field is listed as being mapped by Marketo (under the folder "Field Management" in Field Mapping:

This field that isn't working is listed as an "Unmapped Custom Field" which I'm assuming is simply because it only exists on the SFDC lead record and not the SFDC contact record. Is that right?

Does an SFDC custom field have to be mapped to both the lead and contact to be available as a token?

Thanks!