any objects[via units, destructibles, items, abilities, buffs/effects, upgrades] or triggers or anything that will be manipulated and is common to the melee statistics is considered altered, for example I change the model of the peasant unit, that's now an alter, I made a trigger that shows a game time and score board[this however can be conditional in some cases], now its alter. any small detail that will differentiate from the standard one will be considered altered, but I don't insist you to make such simple edits if your planning to because well its very simple to be called as altered melee.
Except(I guess?) the destructible[scale value and any decorative manuers only, not the life value], some doodads and tile set can be edited mainly because they only decor the map and is being approved by many