Begins optional content belonging to group (/OC /name BDC): visible only while the group is ON.
public void BeginOptionalContent(PdfOptionalContentGroup group)
A group of this document: CreateOptionalContentGroup(String, Boolean), or one read from it.
ISO 32000-2 §8.11.3.2: the group is named in the Properties resources. End it with EndMarkedContent(); optional content nests.