Any constituent field can be pulled via the S1 tag. That means any field that you can see on the constituent's main profile page can be used. The syntax will be:

[[S1:field_column_name]]

where field_column_name is the name of the column header corresponding to that field in reports. This is usually an abbreviated version of the full name in all lowercase with no spaces. Here are some examples of the S1 tag:

[[S1:cons_id]]
[[S1:last_name]]
[[S1:last_trans_date]]