LowGuid and HighGuid

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What does this parameters means ?

[H0zen]

An ObjectGuid is a 64bit number which uniquely identifies an entity (Player, GameObject, Creature, etc.).  Though the only requirement is to be unique, current implementations try to encode as much information as possible into this number, to limit external queries. For example, datas taken from sniffs revealed that the most significant bytes from the Blizzard's GUID identifies the entity type. These bytes were named High Guids (because of their position in the GUID). Low GUIDS are simply entries from databases, or counters, which can help figuring what exact entity is. Technically speaking these High and Low GUIDS have no importance, and it's up to you how you define the ObjectGuid, as long it is UNIQUE.