I could not find this in the documentation anywhere, nor in the forum. We would like to take advantage of the formula capability when mapping fields in the SFDC connector. However, we can't find out if we should be using FA's syntax or Javascript syntax (or something else).
The syntax you use is the Salesforce / FormAssembly / Excel like syntax described in: http://www3.formassembly.com/blog/support-documentation/managing-your-forms/using-formulas/
Most syntax which works for Salesforce / FormAssembly / Excel should work there.
Happy to answer further questions, FormAssembly.com Support.