When some <MetadataWriterPlugin> is used

[RamZesS]

Can somebody explain me when <MetadataWriterPlugin> is used, in which context or conversion process phase?
Is it possible for a user to call it from UI in order to apply current book metadata to the selected book format?

If I provide <MetadataReaderPlugin> for some book format should I develop the <MetadataWriterPlugin> as well?
(I looked up in the code, it looks like not all build-in book formats have both type of plugins)

[kovidgoyal]

writer plugins are used when ever a book is exported from the calibre library via save to disk/send to device/the content server.

[user_none]

The reader and writes are also used by some conversion plugins. Instead of duplicating code I often put the metadata stuff into the reader/writer class and use it in the conversion plugin.

Originally Posted by RamZesS

If I provide <MetadataReaderPlugin> for some book format should I develop the <MetadataWriterPlugin> as well?
(I looked up in the code, it looks like not all build-in book formats have both type of plugins)

You do not need to provide both. It's nice to have both but not aways feasible. As you noted not all formats have both.