What counts as similar for the Jury Rigging perk
For the most part, a simple "if it makes sense" rule of thumb applies. For instance, That Gun can be repaired with basically any non-energy handgun. As a rule of thumb, if it uses the same type of ammo, you can use it to repair another weapon with a similar ammo type, along with a few special cases, like a Hunting Rifle with any lever action gun.
Similarly, Light Armor can repair any other Light Armor, Medium with Medium and so forth.
Items are lumped into groups, and any item can be used to repair any other item from the same group. The Vault has a list here.