How to describe the reverse order which in some contexts is the natural order on things?

In writing a paper my team and I need to explain that in a sublist L of, say, [1,2,3,...,100] (for instance L = [3,5,43,70]) the item with the lowest number has highest priority.

How would one best say this?

Examples where a larger number will denote a lesser priority or importance exist elsewhere:

1. DEFCON1 is the top DEFCON level for instance
2. "priority number 1" describes the highest level of priority

There are a number of possible was to describe how a ranked list, like a list of priorities, is in reverse order. A common solution, in your context of a formal written paper, is to set the scene early in the paper with a sentence like this:

In the following analysis the priority of a XXX is represented by a number in the range 1 to 100, where 1 indicates the highest priority, 2 the next highest, etcetera; with 100 therefore indicating the lowest priority.