What does the cut pattern "farm" mean?

The documentation is a bit sparse on this, I agree. You might want to contact customer support about this. I did some research, and:

• A farm cutPattern is a cutPattern where one or several rows and/or columns of cut are missing every X cuts. For each dimension [X or Y] the same number of missing cut must separate the cut arrays and the same number of cut must be present in the cut arrays.
• Some examples:
  
• If there is no minLargeViaArraySpacing rule in the current constraint group for the current cutLayer the “Farm” mode is not available.

Here's some examples of how it was set up that I found:

stdVia with 3xN pattern
2 missing cuts between array in X (arraySpacingX)
No arraySpacing Y since there is no pattern in Y direction.

Another example:

stdVia with 3x5 pattern
1 missing cut between arrays in X (arraySpacingX)
1 missing cut between arrays in Y (arraySpacingY)

Hope that gives more of a clue about how it can be used?

Regards,

Andrew 

After looking at the rules present in the techno I came to the same conclusion. The farm is a way to maximise via cuts by creating different clusters of via's to fall under a smaller spacing rule. It does this by leaving out rows and/or columns. 

The only piece of information that was missing for, both from the techno and the DRM, was the minimal space between two clusters so they are not interpreted as one. Now there is always one row/col left out. I suppose you need this spacing to decide how many cols/rows you need to blank out.

But besides this remark, is my assumption correct that with this method you always have the max. number of vias?