The number format is the number of digit placeholders to use in the ID.
For example, a number format of "00" will create numbering in the format "01","02","03", etc. A number format of "000" will create numbering in the format "001","002","003", etc.
The number format is very important when it comes to sorting your item IDs. Because sorting is done alphabetically, if you have IDs which are greater than the applied format, the IDs will not sort correctly.
In the above example using "00", if there are more than 99 items, then the IDs would sort as follows:
DOC10
DOC100
DOC11
DOC12
Using the format "000" would sort correctly because of the placeholder zero.
DOC010
DOC011
DOC012
…..
DOC100
Be sure you understand how number formats affect ID naming. In general, it’s best to "over pad" the format with extra zeros, then to "under pad" and have difficulty sorting items.