Well, the specific wording "positional index" actually seems to be introduced in this RFC, but since the text is discussing the syntax for an instance-identifier, which is an XPath expression, and specifically enumerates the allowed XPath predicates in an instance-identifier, it can perhaps be inferred that it is talking about the value of the XPath position() function, see https://www.w3.org/TR/1999/REC-xpath-19991116/#section-Node-Set-Functions. Lists without keys are mentioned in https://tools.ietf.org/html/rfc7950#section-7.8.2 - i.e. lists representing configuration MUST have keys, other lists MAY.
I'm not sure if you are still referring to the syntax of an instance-identifier here... As far as I can see, the value space of the XPath position() function starts at 1. If you are asking about how "lists without keys" are handled in ConfD, see the section "6.11. Operational data lists without keys" in the User Guide. I.e. in short, the application gets to choose a signed 64-bit integer "pseudo key" for each list entry.
Again I'm not sure what you are referring to, but there is no "magic" here. The XPath position() "is what it is". The "pseudo key" is whatever the application has chosen. For the purpose of translating between the textual representation of an instance-identifier and the internal value (which is a hkeypath), ConfD maps any "positional index" to a "pseudo key" with the same integer value and vice versa.