Click on terminal joint.
'alfa' is the rotation of the member about its x-axis i.e. longitudinal axis. For a beam In building frames you would keep it as zero. The alfa is measured in radians. You can change it to change the orientation of columns. The local co-ordinate system of a vertical member with alfa equal to zero is shown below. Shown below is a roof truss purlin with rotated x-axis. Left Click / Right Click on 'a' to assign values equal to 0 / 1.570796
If you want the member not to resist torsional moment then you must release only one of the x,o and x,t. If both ends of a member are allowed to rotate about x-axis then it will become unstable member.
'a' and 'b' are distances from origin joint as shown below.