The difference between masking and clipping is that clipping is 'spatial' and masking is 'logical'. When you clip you define the boundaries of a 3D space and all the members that are physically contained within that space remain active. The clipping bars you see on screen define the boundaries of the clipping box. You can move these clip bars with the mouse or use the 'clip to selection' or 'clip to frame' commands to automatically position the clipping box around the area of interest.
 
Masking allows you to select members to include or exclude from the active list. Mask To Selection makes all the selected members active while Mask Out Selection makes all of the selected inactive. The selection can contain any number of members in any part of the frame.
 
If you mask and clip at the same time, the active member list will comprise those members which will be visible due to the effects of both clipping and masking.

Multiframe adopts the convention that joint 1 is always the joint to the left of the member in the front view and in the case of members which are vertical joint 1 is at the bottom, joint 2 at the top.
 
In the case of a member which lies in the xy plane, joint 1 will be the joint on the left and joint 2 will be the joint on the right. In the case of a member which lies in the xz plane, joint 1 will be the joint on the left as viewed in the right hand view and joint 2 will be the joint on the right as viewed in the right hand view.

The modal shapes created after analysis are non-dimensional and merely reflect the shape the structure would have when vibrating at a given natural frequency. Currently dimensions are shown but the shape is actually scalable to any size. We don't show the induced stresses or actions that result from dynamic analysis as these are not meaningful.

For any continuous structure there are theoretically infinite natural frequencies, but generally engineers dealing with real structures only need to consider those natural frequencies that are likely to occur in the real world. These are usually the lowest natural frequencies for the structure. Multiframe4D calculates from the lowest natural frequency upwards to a possible of 20 natural frequencies.
 
To carry out a modal analysis using computational methods we approximate the continuous system by discretising the structure into a finite number of degrees of freedom. Each degree of freedom allows us to calculate one natural frequency. The more degrees of freedom the more accurate the solution.
 
A good rule of thumb is that the minimum number of degrees of freedom should be at least double the required natural frequencies. Multiframe enforces this rule.
 
Each node has 6 degrees of freedom. A structure with 2 nodes, one fixed, would eliminate the degrees of freedom for that node, so you are left with 6 degrees of freedom in total. Thus considering the rule above, Multiframe4D will only return a maximum of 3 natural frequencies.
 
The solution is to use the subdivide command to increase the number of nodes in the structure and therefore the degrees of freedom. If you experiment with different levels of discretisation you will notice improvements in the accuracy of the solution for the higher natural frequencies as the number of nodes is increased.