Improved Tacks have a thicker shank and a larger head and are used for fixing open weave materials such as webbing and hessians.
Fine Tacks have a smaller shank and head and are used for close weave fabrics.

Guide
10mm - Fine Tacks are used for fitting calico and lightweight top covers.
10mm - Improved Tacks have a slightly thicker shank and a larger head. These are used for medium weight top covers, scrims and bottom cloths.
13mm - Fine Tacks are for fixing heavy top covers, hessians, and webbing on lightweight frames.
13mm - Improved Tacks with their larger heads, are used for open weave fabrics like hessians, and webbing on lightweight frames.
15mm - Fine Tacks are used when fixing thick layers of fabric.
15mm - Improved Tacks are used mainly for webbing.