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First of all, you always have to let the enemy team know which merc you’re using even if one is allowed by default.

Secondly, a lot depends on the tier you’re playing in. Obviously if you’re Open and want to use a default merc who’s played in Mid or above before it’s not allowed unless the enemy team gives you the permission.

Last but not least, you should always look at what competition it is and what game-mode it’s played on. For example if it’s a regular ETF2L 6v6 Season according to Rule 1.4: No mercs allowed unless your opponent agrees.

But in the last Highlander Season (10) the rule used was: 1.4 One mercenary allowed by default and up to 3 mercenaries if the opponent agrees.

I hope I helped clear some things up.

P.S. This is where you can find all the necessary information:
http://etf2l.org/rules/general-rules/
http://etf2l.org/rules/highlander-season-9-rules/
http://etf2l.org/season-25-rules/