Seal Or Entity Emblem Options

I know that a seal of a spirit/entity is used to meditate before evoking/invoking that being, but my question is whats the other uses of the seal ?
Is it have to be placed somewhere in the ritual ? Inside circle of Triangle of Truth ?

This depends on the formula being used. In the lesser key, for instance, the seal of the spirit is worn around the neck as a pendant. No meditation is required, or possibly even helpful although you'd need to do it several times with and without any meditation to decide for yourself. Different books have different requirements though.

The seal can receive enchantments from the entity in question or otherwise serve thereafter as a conduit for the boon the spirit promises. It is dependent largely on the agreement made between the magician and the spirit. Sometimes the seal is the focus of a kind of taboo - it can never see sunlight, or moonlight, or must always be kept in a certain way, etc.

peace