The last three characters identify the probe set strand. Probe sets that are designed to detect the anti-sense strand of the gene of interest are annotated with "_at". Probes in an identical probe set (_s set) all cross-hybridize to the same set of sequences that are used for the design (including any additional pruning sequences if provided). Probes in a mixed probe set (_x set) contain at least one probe that cross-hybridizes with other sequence(s) used for the design.
See tjhe above link for more information, including a nice figure depicting these relationships and definitions.