The mummy / adapted by David Levithan ; retold by Mike Dean. (OCLC #276480317)
A series can have different kinds of subseries: its parts can be named or numbered, and that distinction is indicated by which subfield is used for the parts. Named parts use ǂp, as in:
ǂa 20 questions ǂp History
Numbered parts use ǂn, as in:
ǂa Bitlet. ǂn Series 2
If the part has both a name and a number, both subfields might be used:
ǂa British idealist studies. ǂn Series 3, ǂp Green
Catalogers may disagree on whether a phrase for part of the series is a name or a number. For example, the sets of low-vocabulary books I’m cataloging are in a main series called “Penguin readers” with subseries for the levels of vocabulary: EasyStarts, Level 1, Level 2, etc. I don’t think these really describe a sequence of parts (as mentioned in the docs for MARC field 830) but rather a descriptor of the reading level based on the number of headwords. Headings for these subseries have already been established in OCLC using ǂn for all except the lowest level:
ǂa Penguin readers. ǂp EasyStarts
ǂa Penguin readers. ǂn Level 1
ǂa Penguin readers. ǂn Level 2
....
I am using the form that has been established to reduce future hassle in cataloging. Though it seems odd to me, it should not affect searching or display.