Because the Support article and forum post tells us what channel to use.
Any boosting, carrying, or other similar services offered for gold that’s advertised outside of the Trade (Services) chat channel will be considered spam and the proper actions will be taken, so be sure to switch your service advertisements to the new chat channel when it becomes available after this week’s regional maintenance.
Individuals and guilds selling boost or assistance in raiding, dungeon, or PvP activities for gold is allowed but can only be advertised in-game through the Trade Services chat channel. Advertising for gameplay activities should be done by a WoW player-character who intends to participate in the gameplay activities. This means there should be no cross-realm advertising, and there should be no advertising from non-participating players.