A FieldSig is indexed by the Field.Signature column, or by the MemberRef.Signature column (in the case where it specifies a reference to a field, not a method, of course). The Signature captures the field's definition. The field can be a static or instance field in a class, or it can be a global variable. The syntax diagram for a FieldSig looks like this:
This diagram uses the following abbreviations:
FIELD
for 0x6
CustomMod is defined in §II.23.2.7. Type is defined in §II.23.2.12.