Sometimes when loading a complex schema, you'll need to use the On Demand loading of XML Schema tree feature.
“On Demand (optimized) loading of XML Schema”: If the XSD is complex, then this option allows you to process only the root level element and not the complete XSD. Only when you expand the hierarchy elements, does it processes the child elements and loads them into the memory. The hierarchy elements expansion is now displayed in the applet. This option is applicable only on XML schemas. It optimizes the processing of XML schemas and loads the XSD elements only up to 2 levels in the source and target panels. You need to click a hierarchy element to load its further child elements.
This feature selects the number of levels that can be loaded at a time in memory to prevent the mapper from out of memory issues. When you click on +(expand) in the mapper, it further processes the elements that you have set in “Tree expand level for optimized loading”.