T-SQL - Using Column Name in where condition without any references when having multiple tables that are engaged using multiple joins.

Hi All,

I am a newbie for T-Sql, I came across a SP where multiple tables are engaged using multiple joins but the where clause contain  a column field without any table reference  and assigned  for an incoming variable,like 

where 'UserId = @UserId'

instead -  no table reference like 'a.UserId = @Userid'   ............ Can any please do refer to me any material that clears my mind regarding such issue................... help is appreciated.

Thank You.