Thin yarn categories
The Craft Yarn Council lists sock, fingering and baby among the names in category 1, Super fine. Sport and baby appear in category 2, Fine. The guide also lists fingering among category 0 Lace names, so a name alone is not an exact category.
Read the label
Check the category number, label length and suggested gauge. Regional ply names are conventions and do not necessarily count the strands twisted into a yarn. See ply.
If changing yarn, use a gauge swatch in the actual stitch pattern. Equal ball weights can contain different lengths.