In certain circumstances it may be convenient to create .ufm files of “partial” matrices which contain only a subset of the matrix, in particular a subset of the rows. See, for example, Section 20.6.19 where skimming may be carried out on a restricted set of origins in order to make use of distributed cores and/or threads to do the calculations in parallel.
Such .ufm files should behave exactly as full matrix .ufm files in terms of viewing or manipulating them in MX although probably the most common operation in such cases will be to stack them into a full matrix using STACK; see 10.20.11.
In the limit a partial matrix could consist of a single row, in which case it would be effectively a vector which opens up the possibility of using the .ufm structures to store row-based data such as trip ends. One for the future.