NOMINATE is a two-axis score, with dimension A representing traditional liberal/conservative split on economic issues, and dimension B representing responses to a contemporary slate of social issues (civil rights, immigration, etc). By adjusting the dimension weighting, you can change how much each counts in generating an aggregate score.
NOMINATE looks at congresspersons over their entire voting record, so for each year, the congressperson's score reflects votes that they have cast in the past (or will cast in years later than the one recorded). Alternatively, Nokken-Poole scoring treats each session of congress, and the congressperson's votes in it, as a separate category.