When a conjurer performs the act that invites spirits it can be compared to moths to a flame. The energy that is placed out there by the conjurer attracts spirits and typically is one act of conjuration no less than three spirits respond within a matter of seconds and want to be pulled. The choice of spirit is left to the discretion of the conjurer.
Typically criteria for a spirit to be a good candidate for binding is their energy, their presence, their demeanor and their abilities. The strongest spirit will always present itself first but depending on the purpose for the binding and the Keeper the conjurer is binding for the first may not always be chosen. If a conjurer is performing a binding specifically for another Keeper the Keeper's needs and personality are always at the forefront of making a spirit choice.
Spirits exist in the thousands around all humans at all times and literally hundreds of billions exist within the three realms it is choosing the proper spirit to bind that is the art and talent of a conjurer.