There's more to a 1st line than a belt and gear associated to that belt. Sometimes a war-belt can Ben be considered to be part of the 2nd line of gear.
1st line is basically a persons essential gear, including but not limited to; every part of ones uniform/clothing(underwear/socks, boots, etc.), essential navigation gear, personal weapon(s), etc.
Of course, some of the gear can fall into the 2nd or 3rd lines of gear as well. While 2nd line gear consists mainly of ones fighting load, some may spread out their 2nd line to include a belt kit and not only a plate carrier, load bearing vest, chest rig, or whathaveyou.
There's a lot of information to be found on this subject, but that's the basics of some of it in my words.