The bounty tasks at the Mercenary Guild are generally public, and anyone with sufficient strength can take them on, of course, you can also choose to take on those high-difficulty missions if you're not afraid of death, but the problem is, you have to be able to come back alive.
As for the covert missions, only bounty hunters of A Grade or above can take them, these tasks generally arise from minor frictions and subsequent conflicts between underground forces or official powers that are unable to deal with each other on their own.
However, driven by mounting anger, some factions choose to retaliate against their high-status enemies by issuing bounty tasks through the Mercenary Guild. The guild has always been a public organization, without any private undertakings; anyone can hire a bounty hunter from the guild, provided they can afford to pay enough Yajin or gold.