Separate the topics by prefixing individual files with C, T, R, G, and M for creating Concept, Task, Reference, Glossary Entry, and DITAmaps respectively.
For example:
C_dita_topic_types.concept.dita
T_how_to_set_java_environment_variables.dita
R_what_is_localization.dita
G_meaning_of_terms_used.dita
M_fundmentals_of_dita_translation.ditamap