|DITA map files / Relationship tables|
Even for medium-sized documents, it is easy for ditamap files to become unwieldy due to the number and complexity of topic references. You should take advantage of the modular nature of DITA to store relationship tables in separate ditamaps, embedded within the main ditamap, as soon as the ditamap becomes too large.
For collections larger than around 50 topics, it is preferable to store relationship table information in a separate ditamap file, embedded (nested) within the master ditamap for the collection. The relationship table map should only contain reltable sections.
For very large documents, you may choose to have multiple embedded reltable ditamaps.