Pending Write Back

[P.Jones]

Why does IM not recognise when a file has not been changed?

I select some files (actually I selected 132 files), I then select a new category for these files, decided I made a mistake so deselect that category .

The files have not changed.

So why does IM mark them all with a yellow pencil indicating that the files are now 'Pending write back'

My request, please fix this.

[Mario]

You added a category. Then you removed a category. The file is changed. IMatch cannot track your changes and somehow magically recognize that the two independent category assignments you have made cancel each other out.

Did you try to use Edit > Undo instead?

[P.Jones]

You added a category. Then you removed a category. The file is changed. IMatch cannot track your changes and somehow magically recognize that the two independent category assignments you have made cancel each other out.

Yes I selected a category, this was not written to the file, and then deselected the category. So the file is unchanged

Did you try to use Edit > Undo instead?

Tried that but the yellow pencil remains pending write back.

[Carlo Didier]

Yes I selected a category, this was not written to the file, and then deselected the category. So the file is unchanged

You are right (the file hasn't changed on the disk), but wrong too (categoriy assignments have changed twice).
Although, no changes have been made to the files on disk, you did two independent category assignment changes. Any change to metadata that is not automatically written to the files is signalled by the yellow pencil. That icon does not say that the files have changed, but that metadata has been changed but not written to the files.

[Mario]

Changing a category does not cause a write-back, unless you change @Keywords categories - which are automatically written to the in-database XMP keywords. And hence cause the file to switch into the 'pending write-back' mode.

[sinus]

If I drive 100 m with a car, and drive then back exactly to the starting point, it is also like before ... nothing has changed?! 

This is digital world, in what we are living now, not more the old analog one. A lot seems not logical, but in the end it is.

[Carlo Didier]

If I drive 100 m with a car, and drive then back exactly to the starting point, it is also like before ... nothing has changed?! 

Well, no, of course! 
You have used time and fuel and your car got some wear ...

[P.Jones]

As an aside to my original post I had a look at the new  'File Categories Panel'.

Using this I unticked a category on a photo. This removed the category from the photo but no yellow pencil was shown.

Does this mean that the modified file is updated automatically and straight away,

i.e. works differently to the 'Categories panel'

[Mario]

Only when you work with @Keyword categories you change metadata as well. If you change other categories, this will not cause metadata updates.