Purpose – To inform the user of foreseen additions to the unit of description.
Rule - Indicate if accruals are expected. Where expected, give an estimate of their quantity and frequency.
It is probably best to follow past practice in populating this field. Users will still find it useful to know if more material is going to come in at a later date. On a side issue, it should be noted that experience with born digital accruals to date has shown that you are much more likely than with paper, to get duplication with earlier accessions, e.g. a later snapshot of the same hard drive will have much of the same content.