2D vector (often used to store positions, sizes, etc.)

4D vector (often used to store floating-point colors)

Runtime data for multiple fonts, bake multiple fonts into a single texture, TTF/OTF font loader

1D vector (this odd construct is used to facilitate the transition between 1D and 2D, and the maintenance of some branches/patches)

Storage for one type registered in the .ini file

Helper to build glyph ranges from text/string data

All draw command lists required to render the frame + pos/size coordinates to use for the projection matrix.

Helper to build a ImDrawData instance

Storage data for a single column

Backup and restore IsItemHovered() internal data

Storage for a tab bar

Storage for a tab item (within a tab bar)

Temporary storage for one window (that's the data which in theory we could ditch at the end of the frame)