X/Y means array in the x/y-direction, S/E means starting and ending rows. These are values of U at the valid boundary time. Where there is a letter T in the field name, that means the change of the field in the time period up to the next valid boundary time. The following site has a document regarding the lateral boundary:
http://www.mmm.ucar.edu/wrf/WG2/software_2.0/index.html
http://www.mmm.ucar.edu/wrf/WG2/software_2.0/index.html