By default, your projects are assigned a project number with the format P-(number) starting at P-100 and going up.

If you'd like to customize this numbering, just go to Settings > Company > Numbering.

Here you can assign a new prefix for numbering your Projects and Purchase Orders, and choose which number to generate next.

If you are an Enterprise user, you also have the ability to use a different prefix for numbering the projects made by each Office in your account. Go to Settings>Office>[Select your Office name]>Numbering, to enable unique office numbering.
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Once this is enabled, you can specify a new identifying prefix to go with the existing number scheme that is set under Company>Numbering.

Hope this helps - let us know if you have any questions!