Schematic layer properties are the set of layers properties that are specified for each feature layer that compose a schematic diagram layer. They are specified within ArcMap from the schematic layer and its related feature layers and may end with very different visualizations for each diagram.
Learn more about schematic layer properties
Default schematic layer properties are the set of layers properties specified at the schematic diagram template level to be used as default properties for all the diagrams based on that schematic diagram template. Specifying default schematic layer properties on a diagram template allows you to get all schematic diagrams based on that diagram template represented with the same schematic layer properties so the diagrams display in the exact same way.
Learn about defining default layer properties for a diagram template