The focus is print a Technical Manual? The basic information exists and is being imported OR is in the process of being created?
Computers sort in a rigid manner - the worst case senario often defines how to construct fields that are used for manipulating the records, separate from the labels in the final document. Section 1, Section 1.1, Section 22.214.171.124, Section 123.999.999. Defining the outlying parameters.
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I suggest that you make each field such that they only contain a single number. This will make sorting on them much simpler.
Thus, a record with the values 1.2.3 would have
Primary Index: 1
Secondary Index: 2
Third level index: 3
A calculation field can combine these fields for search and display purposes. You can also put the three fields as merge fields on a layout to display the combined values as a single composite index.
I thought FileMaker handled all indexes automatically or does it do it automatically in the field order? I like your concept except it must fit, like everything else, into the big picture.
Attached (audit.png) is a screen shot of a Word doc or PDF that will be needed at the end of the audit as a report and should have the same basic look. As it shows the placement of the numbered sections along with the associated text which is placed in other fields will be interesting. The way I have it setup now provides the ability to Find each group within a number secgment, if your process does this I'm all for it.
In FileMaker terms, these aren't indexes, but simply numbers in number fields. But with careful design, including auto-enter calculations and possibly scripts, the result you show is possible, but keep in mind that FileMaker isn't a word processing or desktop publishing program. Some things, that building relational links between different records that make up your tech document will be easier and enable you to do things you can't do in word, but formatting, page breaks and the outline style item numbering are not something that FileMaker has built in tools for and some formatting issues such as "clean" page breaks with no "widows" or "orphans", it can't do at all.