Classic triggers have no variables. What they do have is a slot per player per unit type recording how many of that unit died - and triggers can read and write that slot freely. So map makers use it as a variable.
SetDeaths(CurrentPlayer, SetTo, 0, "Level"); SetDeaths(CurrentPlayer, Add, 1, "Level"); Deaths(CurrentPlayer, AtLeast, 100, "Level");
“Level” here is a unit type name. In ScmDraft you rename a unit type you never use to Level and place none of them on the map. That slot then belongs entirely to you.
If a unit you are using as a counter actually dies during play, the value climbs on its own and your variable is corrupt. That is why it must be a unit type the map never spawns.
A computer player's death counters make good globals. Per-player values go on CurrentPlayer; values the whole map shares go on P8.