What is the year in this circumstance?

If I understand your question the use of 1990 is due to the example itself and not that year in particular. They are using a year in the past to compare and make a point about more recent years.

The use of the year 1990 is also detailed in the answer under number 2.

"...Consequently, they also mean it in more narrow way that it would mean that in their regression of margins on those leniency laws the leniency laws would be correlated with error term. In fact, this is why Dong says that they dealt with it by using leniency index from 1990s - using past ('lagged') values instead of contemporaneous one's is a common econometric technique to deal with endogeneity in an empirical model."

The use of multiple subscripts chasing around various meanings goes back to Einstein's General Theory of Relativity where he used tensors and came up with a few shortcut conventions of necessity.