Air Control
There are two server variables that affect the amount of movement control that players have while off the ground: sv_aircontrol and compat_limited_airmovement. Technically, compat_limited_airmovement false changes the way sv_aircontrol is interpreted, but in practice it is almost always used with the default air control 0.00390625.
Common Settings
For mods comprising mainly new maps for Doom II, these three are the most common server configurations for air control:
Doom
sv_aircontrol 0 compat_limited_airmovement true
Used for most competitive Capture the Flag games, this is the behavior in vanilla Doom. Players cannot change their direction at all while in the air, save via some external source (such as an explosion).
ZDoom
sv_aircontrol 0.00390625 compat_limited_airmovement true
This is the default air control for ZDoom. While such a small amount (1/256th) is hardly enough to make any meaningful movements while off the ground, it makes jumping onto ledges substantially easier.
Skulltag/Zandronum
sv_aircontrol 0.00390625 compat_limited_airmovement false
This was the default setting for Skulltag and is now for Zandronum. Air control is still low, but airborne players can accelerate beyond their normal top speed: by “bunnyhopping”, players can achieve great linear speeds, although their ability to change direction is diminished while in the air. Many Skulltag-based mods require this setting.
(Un)Limited Airmovement
Using sv_aircontrol 1 with compat_limited_airmovement true, players can accelerate and move around in the air just as well as on the ground; however, they cannot accelerate beyond their normal running speed because “air friction” slows them.
When compat_limitedairmovement is disabled, players' air control is always 0.25; but their air friction is unaffected; therefore, when sv_aircontrol is the default 1/256th, their actual air control is a whopping 0.25 while air friction is negligible, allowing them to reach incredible speeds in the air. Increasing the value of sv_aircontrol will have no effect on players' actual air control, but it will increase air friction, actually slowing them down!
Flight
When given the power of flight, players use their ground speed in the air, so modders usually need not worry about what air control does in this instance. For flying playerclasses, however, it can sometimes be possible to use the land command to gain extra speed. To prevent this, server hosts can use the flag Compat_NoLand true, thereby disabling the land command entirely.