Updates a single named field on an existing maintenance sub type record, identified by `maintenanceSubTypeId`. 
Unlike `updateMaintenanceSubType` (which requires the full set of fields), this function updates only the one column named in `field`, setting it to `value`.

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### Parameters
**maintenanceSubTypeId** `Integer`: Id of maintenanceSubType to update<br />
**field** `String`: Valid options are title, maintenanceSubTypeName, maintenanceTypeId, enabled<br />
**value** `String`: Value to update to<br />
**userId** `Integer`:Id of the user executing the function<br />

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### Returns
**# of records modified** `Integer`<br />
**message** `String` None if success<br />

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### Example
```python
# Update just the name of an existing maintenance sub type
maintenanceSubTypeId = 14
field = "maintenanceSubTypeName"
value = "Belt Replacement"
userId = system.kanoa.security.getIDPUserId({'userName': 'SYSTEM'})
recordsModified, message = system.kanoa.cmms.updateMaintenanceSubTypeField(maintenanceSubTypeId, field, value, userId)
print recordsModified, message
```