The “What profile” drop down shows Profile Select / Search attributes that can be selects. Those are the attributes that would hold a Profile as a value so you pull values from that Profile.
If you are running this Update workflow on a Student Profile already, and you want to set “S_Student” to the same value of something that already exists, you could use the “another attribute’s value” option under “Set as”
Or, if you are running this from a different profile, you would need to set the Student Profile in a Profile Select / Search Attribute and then select that attribute from the “What Profile” list.