Add clarifying comments to pre-stage filter macros

dbt/macros/pre_stage_filters.sql

@@ -3,13 +3,22 @@
 --
 -- Note that the `cur = 'Y'` filter is not necessary in the
 -- default.vw_pin_value query that is the main consumer of these macros, since
--- that query already filters rows where `cur = 'Y'`
+-- that query already filters rows where `cur = 'Y'`.
+--
+-- A PIN is in the pre-mailed stage for a year if none of its representations
+-- in `asmt_all` in that year have procnames (i.e. stage names). We can check
+-- for this status by checking the cardinality of the `stages.procnames` array,
+-- where "cardinality" is the Trino SQL equivalent of the length of the array
 {% macro pre_mailed_filters(tablename) %}
     {{ tablename }}.procname is null
     and {{ tablename }}.cur = 'Y'
     and cardinality(stages.procnames) = 0
 {% endmacro %}
 
+-- A PIN is the pre-certified stage for a year if it has at least one
+-- representation in `asmt_all` with a procname (i.e. stage name), but none
+-- of its representations in `asmt_all` have the procname that corresponds to
+-- the Assessor Certified stage
 {% macro pre_certified_filters(tablename) %}
     {{ tablename }}.procname is null
     and {{ tablename }}.cur = 'Y'