If you source a publication online which is also available in hard copy, cite the hard copy version. There is no need to cite an electronic source for such a publication.
More difficult is the case where no volume number is given.
Safet Orucevic, ‘Mostar: Europe’s Failure’  2 Bosnia Report 25.
Alison L Young, ‘In Defence of Due Deference’ (2009) 72 MLR 554
You might describe the "2" as an issue number or as a volume number, but its role in the cite is the same either way. The "2" should go in the Volume field.
I don't know how you can discriminate between them in CSL 1.0.1, without resorting to some sort of style-specific workaround
Luckily, I can fix this particular issue by manually editing the volume number from 4 --> 4(3)
Random secondary sources (item type=document)
I don't think it's style-specific, but the workaround here is that we assume year-as-volume where we have a year but no volume number.
1. Henry G Schermers and Niels M Blokker, International Institutional Law: Unity within Diversity (5th edn, Martinus Nijhoff Publishers 2011)
1. Schermers and Blokker (n 1)
Motoyo Kamiya, ‘A Study of Formal Relationships between Civil Society and Multilateral Bodies: Accreditation and Other Consultative Modalities’ (2007)
Motoyo Kamiya, ‘A Study of Formal Relationships between Civil Society and Multilateral Bodies: Accreditation and Other Consultative Modalities’ (Building Bridges III: Engaging Civil Society from Muslim States and Communities with the Multilateral Sphere, 27-28 October 2007)
A Tsadiras (2012), ‘The Ombudsman’, in P Craig, EU Administrative Law (2nd edn, OUP).
1. Alexandros Tsadiras, ‘The Ombudsman’, EU Administrative Law (2nd edn, OUP 2012).