How to differentiate whether a field modification is done by UI or REST API ?

In your smart list you will use the data value changes trigger and you can add the constraint called reason and set the value to "Manual Lead Update".

HI,
Thanks for the reply. Not sure why, But I did not get this value in auto fill suggestions.  I added reason "Manual Lead Update" manually when the "First Name" data value changes in my smart list. But still, A lead did not get triggered when i update the first name. any suggestions? 

Are you setting the name for the fist time? Or changing it from X to Y?

HI,
I am changing the first name from X to Y. Here is whole scenario:
  If a new lead is created or any modification to the existing lead on First Name, email.. etc  - calls webhook to push the lead details External (Siebel) System using a webhook. Webhook is working fine when invoked for a new lead/modification to lead attributes. Again Siebel will cleanses contacts and updates leads in marketo using the REST api with UUID Details. To Avoid infinite looping wanted to differentiate modifications done by UI and API. 

Santhosh - I do not understand why this would create an infinite loop? Here's my understanding:

The first webhook would be called based on a data value change. You then update the field via the API. This would trigger a second webhook, but on the second API call the data you are pushing into Marketo would be the same. So there would be no change data value the second time. You would then exit the loop because a third webhook would not be called.

HI, Its not necessarily the same data from SIebel for the already existing contacts. Everytime External system updates the lead data( ex: first name AND/OR address.... )  using REST API in Marketo, The webhook is re-triggering as data is updated by REST.  Where can I get more info/docs on "Manual Lead Update" reason?