Runtime IDs have a relatively short lifetime, at most a Blender session, but very often much shorter, if used e.g. as temporary work data during some operation.
This data is also pending ID Management: Sanitize and clarify our ID tags and related tasks, which among other things aim at clarifying the temporary IDs types and management.
There are four main variations of runtime IDs currently in Blender:
- Evaluated data, tagged with
LIB_TAG_COPIED_ON_WRITE_EVAL_RESULT: The depsgraph stores the results of its evaluation in temporary data-blocks, which lifetime and allocation/freeing are managed by itself. Those evaluated IDs can share some data with their original version, especially heavy sets of data or caches.
- Fully out-of-main, independent data, tagged with
LIB_TAG_NO_MAIN: Those are fully independent data generated during some processing, they should never share any data with any other ID. Those are 'convertible' by generic code into regular 'in Main' IDs.
- IDs from temporary Mains, tagged with
LIB_TAG_TEMP_MAIN: Those IDs essentially behave like regular data-blocks, and follow the same rules. But they do not belong to the Main instance representing the current .blend file, but to some other, temporary data-base.
- Localized IDs, tagged with
LIB_TAG_LOCALIZED: Currently used for some ID types during rendering process. To be precised/refined/extended.