Syncing a SFDC custom field that is not on the Lead, Contact, or Account object.
Any custom field created in Salesforce that is not linked with Lead, Contact or Account objects will not sync to Marketo. Only the Lead, Contact and Account objects can be synced down to Marketo by default. If you would like to have any custom field synced to Marketo, it must be associated to a Lead, Contact or Account object in Salesforce.
Custom Objects created in your Salesforce instance can be part of Marketo too. To use a custom object in Marketo, it must be associated to a Lead, Contact or Account object in Salesforce as well. For more information on syncing Salesforce Custom Objects, please see the documentation here: SFDC Sync: Custom Object Sync.
Fields on the SFDC Custom Object do not become fields in the Marketo lead database, but can be used as filter constraints on Custom Object filters - Add/Remove Custom Object Field as Smart List/Trigger Constraints.