Add a running number column based on a text value

Transact-SQL
4

@mike01 - Also, is it possible to do a similar thing for records found before "Header1" for each ID and mark all such records as 0? following is the new source and desired tables. please help.

create table #SourceTable (UniqueKey int identity(1,1),ID int, SomeText varchar(50))

insert into #SourceTable values (1111,'someRandomText')
insert into #SourceTable values (1111,'someOtherRandomText')
insert into #SourceTable values (1111,'NonHeaderText')
insert into #SourceTable values (1111,'Header1')
insert into #SourceTable values (1111,'dtl1')
insert into #SourceTable values (1111,'dtl2')
insert into #SourceTable values (1111,'dtl3')
insert into #SourceTable values (1111,'Header2')
insert into #SourceTable values (1111,'dtl1')
insert into #SourceTable values (1111,'dtl2')
insert into #SourceTable values (1111,'Header3')
insert into #SourceTable values (1111,'dtl1')
insert into #SourceTable values (1111,'dtl2')
insert into #SourceTable values (2222,'someRandomText')
insert into #SourceTable values (2222,'Header1')
insert into #SourceTable values (2222,'dtl1')
insert into #SourceTable values (2222,'dtl2')
insert into #SourceTable values (2222,'Header2')
insert into #SourceTable values (2222,'dtl1')
insert into #SourceTable values (1111,'someOtherRandomText')
insert into #SourceTable values (1111,'NonHeaderText')
insert into #SourceTable values (9990,'Header1')
insert into #SourceTable values (9990,'dtl1')
insert into #SourceTable values (9990,'dtl2')

select * from #SourceTable
drop table #SourceTable

and desired table is as following

create table #DesiredTable (ID int, SomeText varchar(50),Identifier int)

insert into #DesiredTable values (1111,'someRandomText',0)
insert into #DesiredTable values (1111,'someOtherRandomText',0)
insert into #DesiredTable values (1111,'NonHeaderText',0)
insert into #DesiredTable values (1111,'Header1',1)
insert into #DesiredTable values (1111,'dtl1',1)
insert into #DesiredTable values (1111,'dtl2',1)
insert into #DesiredTable values (1111,'dtl3',1)
insert into #DesiredTable values (1111,'Header2',2)
insert into #DesiredTable values (1111,'dtl1',2)
insert into #DesiredTable values (1111,'dtl2',2)
insert into #DesiredTable values (1111,'Header3',3)
insert into #DesiredTable values (1111,'dtl1',3)
insert into #DesiredTable values (1111,'dtl2',3)
insert into #DesiredTable values (2222,'someRandomText',0)
insert into #DesiredTable values (2222,'Header1',1)
insert into #DesiredTable values (2222,'dtl1',1)
insert into #DesiredTable values (2222,'dtl2',1)
insert into #DesiredTable values (2222,'Header2',2)
insert into #DesiredTable values (2222,'dtl1',2)
insert into #DesiredTable values (1111,'someOtherRandomText',0)
insert into #DesiredTable values (1111,'NonHeaderText',0)
insert into #DesiredTable values (9990,'Header1',1)
insert into #DesiredTable values (9990,'dtl1',1)
insert into #DesiredTable values (9990,'dtl2',1)

select * from #DesiredTable
drop table #DesiredTable

Highly appreciate your help with this on. Thanks @mike01