General formatting of hologram line content.
There are multiple different types of Hologram Line content. If other than text, the type must be defined in the lines content. (#<type>: ...)
Icon line is just a floating item. You can use any material, create player heads, use PAPI placeholders or modify NBT data.
Of course, you don't have to enter all of the parameters. Here are a few examples:
Head lines are displayed as a helmet of normal-size armor stand.
Some Materials and Entity Types will only work on certain versions. For example, you can't use Entity Type AXOLOTL on 1.8 server.
There are three ways you can create player heads.
This would display a different head to each player.
You can use a Base64 string textures.