Where did 00-09 go?
This qualifies as an "advanced concept". You can get along just fine with Johnny.Decimal without knowing about this.
The astute among you will be wondering why we started at 10 and not 00, and then why we started at 11 rather than 10.
We reserve these for the system
Sometimes you want to keep information about the system, in the system. I call this Management & Meta.
'Meta' basically means 'self-referential'. It's about itself. Get it? If not, don't stress.
00-09 is about the Johnny.Decimal system itself
I always reserve these numbers for a special area called 00-09 Management & Meta. Inevitably I want to store some information in my Johnny.Decimal system about my Johnny.Decimal system. That goes here.
For example, I keep text notes in a folder on my disk. That folder has to live somewhere, and it lives in a folder here.
One of the text notes I keep at work serves as a quick-reference index to my Johnny.Decimal system. It's called 00.00 Index.
So 00-09 is an area which is used to manage the system. Hence 00-09 Management & Meta.
10, 20, 30 are about those categories
In the example we've been using here, 20-29 is Contracts, but our first category was 21 Company registration. What happened to 20?
We reserve these numbers for 'management & meta' information about the category. Now, I hardly ever use them; but it's rare that I have more than 9 categories in an area, so I don't miss the number that I've wasted.
Confused? Don't worry about it
If you want to use the zeros, use them. It doesn't matter.