There are several yarn weight systems around the world which make comparing yarns a little confusing. We have decided to adopt the US standards (more on why a little later).
Craft Yarn Council (www.yarnstandards.com) categorises yarn weights from lace to (0) Super Bulky (6).
The reason Thread Collective has adopted the US standards is that in Australia the yarn guidelines refer to 2 Ply, 4 Ply, 10 Ply etc. The use of different rules creates much confusion, especially when it comes to yarns that can be used for weaving as weaving yarns introduce a whole other measurement system. The issue with the Australian Ply system is that as an individual ply of one thread can be finer than a 4 Ply of another brand.
We get asked the question regularly and would recommend you look at the meters/100g of the yarn recommended in your pattern and compare for a similar measurement in other brands.
If you would like more information of understanding what the numbers in weaving yarns (ne 8/2, Ne 8/16) are I would suggest you read the article 'Understanding Yarn Count in Weaving'.