I've been keeping a chart of differences between custom fields, custom objects, and custom activities to help my customers choose the right place to store data. Not all of the functionality is immediately obvious but it can have a big impact down the line.

 

 

Custom Field (person)

Custom Object

Custom Activity

Can be used to personalize email

X

X

 

Can be used to personalize mobile

X

 

 

Can be used to segment/target/trigger off value upon insertion

X

X

X

Can be used to segment/target off current values

X

X

 

Can be used to trigger off changes to an attribute

X

X (Since 2017)

 

Can be 1:N with a person

 

X

X

Can be aggregated over time period (frequency)

 

 

X

Can be created by a Marketo admin

X

X

X

Can be added (individual entries) via SOAP API

X

X

 

Can be added (individual entries) via REST API

X

X

X

Can be programmatically updated and deleted (individual entries)

X

X

 

Can utilize text attributes (>255 char)

X

X