A document schema is used to define a message. It is a definition on an Xml message with optional extensions for flat files, EDI file, etc that enable the parsers to convert the native format into Xml.
A property schema is used to define message context properties. These can be of type MessageDataPropertyBase (the property value is promoted or demoted from/to the message itself) or MessageContextPropertyBase(property value only exists within the message context and can be set by adapters, pipelines or within orchestrations).
If you wish to promote a field from a message into the message context then you need to define a document schema and property schema. In the document schema you promote the required field using the property schema to define the property type that will be used in the message context.