Sidedef
From ZDoom Wiki
A sidedef is a structure bound to a linedef in order to specify for one side of the linedef which sector to refer to, what texture to use and where, and how to display and offset that texture. Sidedefs are assigned to linedefs, not the other way around. Just like sector data, sidedef data can be reused for multiple lines or sets of lines. Where a given sidedef's properties are the same as those of another sidedef, either sidedef can be used in either place, and the redundant sidedef can be discarded. This can significantly reduce the filesize of a saved map, and is known as sidedef packing. ZDoom unpacks these sidedefs when maps are loaded, however, presumably for stability purposes.
Specification
| Bytes | Data Type | Description | Notes |
| 0-1 | Signed Short | X offset | |
| 2-3 | SIgned Short | Y offset | |
| 4-11 | Char | Upper texture | |
| 12-19 | Char | Lower texture | |
| 20-27 | Char | Middle texture | |
| 28-29 | Unsigned Short | Sector reference | This must be correct for sectors to be closed |
