Unlike more mechanistic systems wherein spell effects are defined rigidly by area/volume, Sorcery in Vivid defines the area of effect in terms of delimitations.
The basic idea is a spell effect is created by spirits whom you have somehow persuaded to do as you will. However, their capacity to take instruction from you is somewhat limited - it's like trying to give complex instructions in English to a foreigner child who barely understands it. Thus, instructions have to be given in very easy to understand terms. When defining an area for effect, it's best to physically mark out the area you want affected beforehand, or during the casting of the spell.
For example: I want a ring of fire to spring into being when my enemies enter this place, thus trapping them. To define the limits of the ring, I draw it out using powdered charcoal - as an inflammable material it's very appropriate for a fire spell, and it's easy to tell the fire spirits to create the effect using the physical traces of the charcoal as their guide.
What if no delineation is provided? Then the spell takes effect as the spirits want - possibly too little, possibly too much, very possibly with complications! Hoo hoo!