Set Person field type in a SharePoint Designer Workflow best practices

Question

In SP Designer, when using the "Set Field in Current Item" action, if the field you are setting is a Person type, there are several choices to "return" the data as. For example, if setting your Person field to the current item's Modified By or Created By, these are also Person fields but can be returned as one of the following:

• As String
• Display Name
• Email Address
• Login Name
• User ID Number

Is there any gotcha in choosing one of these to set a Person field as? Is there any choice that won't set the person correctly and resolve it to a SharePoint person type?

Answer 1

I always use the User ID Number since that will always be unique for the user. Depending on the set up and management of users the Display Name, Email Address and String may not be unique.

Answer 2

I always use "Login Name" since that will always be unique for the user within the SharePoint farm. User ID Number proposed by Ryan is ID from hidden SiteUserInfoList and may differ between Site Collections. Also you cannot use Send Email workflow action using User ID Number.