with t(srt,val) as (
  select 1, 'A' from dual union all
  select 2, 'S' from dual union all
  select 3, 'S' from dual union all
  select 4, 'A' from dual union all
  select 5, 'S' from dual union all
  select 6, 'S' from dual union all
  select 7, 'A' from dual union all
  select 8, 'S' from dual
 )
select  srt,
        val,
        case val when 'S' then g else 0 end g
  from  t
  match_recognize(
                  order by srt
                  measures
                    match_number() g
                    all rows per match
                    pattern(strt s+)
                    define
                      s as val = 'S'
                 )
/
       SRT V          G
---------- - ----------
         1 A          0
         2 S          1
         3 S          1
         4 A          0
         5 S          2
         6 S          2
         7 A          0
         8 S          3

8 rows selected.

with t(srt,val) as (
  select 1, 'A' from dual union all
  select 2, 'S' from dual union all
  select 3, 'S' from dual union all
  select 4, 'A' from dual union all
  select 5, 'S' from dual union all
  select 6, 'S' from dual union all
  select 7, 'A' from dual union all
  select 8, 'S' from dual
 )
select  srt,
        val,
        case val when 'S' then g else 0 end g
  from  t
  model
    dimension by(srt)
    measures(val,0 g)
    rules(
          g[any] order by srt = case
                                  when val[cv()] = 'S' and nvl(val[cv() - 1],'X') != 'S' then  nvl(g[cv() - 1],0) + 1
                                  else nvl(g[cv() - 1],0)
                                end
         )
/
 

       SRT V          G
---------- - ----------
         1 A          0
         2 S          1
         3 S          1
         4 A          0
         5 S          2
         6 S          2
         7 A          0
         8 S          3

8 rows selected.