These can be a good way of getting your work seen by ‘opinion formers’ or commissioners and may even lead to you being selected or contracted for a commission. Registers and indexes vary enormously, some are selected or invited artists only, others are completely open. Slide registers may cost in one way or another. Some have registration fees, others are free to join but you’ll need to provide the slides for the register to keep indefinitely. In particular in recent times internet based directories have become far more useful.