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]]

You can field the fields by editing a constituent profile and then inspecting the HTML. The field that can be pulled by the S1 tag is defined as the input_id. For example, the HTML for first name is

<input name="first_name" id="first_name" value="Alexandra" maxlength="50" size="13" type="text">

Where id equals the label that can be used for the S1 tag (first_name).