This table summarizes the various types of objects that every combination of flags for $lfn[llSensor] and $lfn[llSensorRepeat] is able to detect. Combinations of $lct[AGENT], $lct[ACTIVE] and $lct[PASSIVE] without $lct[SCRIPTED] are straightforward. $ct[SCRIPTED] can only be combined with $ct[PASSIVE].

The right column indicates the type that $lfn[llDetectedType] returns for the kind of detected object indicated in that row. It has nothing to do with the parameter passed to $fn[llSensor], even if the constants are common. Blame LL for the confusion this causes ;)

^ $lfn[llSensor] flag ^ Types of objects that this flag can detect ^ What\\ $lfn[llDetectedType]\\ returns ^
| $ct[AGENT] (1) | • Agents not sitting on an object (i.e. either sitting on ground, or not sitting at all). | $ct[AGENT]`|$ct[ACTIVE] |
| ::: | • Agents sitting on an object (but not on ground). | $ct[AGENT]`|$ct[PASSIVE] |
| $ct[ACTIVE] (2) | • Unscripted physical objects that are moving. | $ct[ACTIVE] |
| ::: | • Physical objects with active scripts (*), or scripted physical objects that are moving. | $ct[ACTIVE]`|$ct[SCRIPTED] |
| ::: | • Non-physical objects with active scripts. (*) | $ct[PASSIVE]`|$ct[SCRIPTED] |
| $ct[AGENT]`|$ct[ACTIVE] (3) | Includes everything that $ct[AGENT] and $ct[ACTIVE] detect separately. | See above. | 
| $ct[PASSIVE] (4) | Detects only the objects that $ct[ACTIVE] doesn't detect. ||
| ::: | • Physical non-moving unscripted objects. | $ct[ACTIVE] |
| ::: | • Physical non-moving objects with an inactive script. (*) | $ct[ACTIVE]`|$ct[SCRIPTED] |
| ::: | • Non-physical unscripted objects. | $ct[PASSIVE] |
| ::: | • Non-physical objects with an inactive script. (*) | $ct[PASSIVE]`|$ct[SCRIPTED] |
| $ct[AGENT]`|$ct[PASSIVE] (5) | Includes everything that $ct[AGENT] and $ct[PASSIVE] detect separately. | See above. |
| $ct[ACTIVE]`|$ct[PASSIVE] (6) | Includes every object (everything that $ct[PASSIVE] and $ct[ACTIVE] detect separately).  | See above. |
| $ct[AGENT]`|$ct[ACTIVE]`|$ct[PASSIVE] (7) | Includes every object and agent (everything that each flag detects separately). | See above. |
| $ct[SCRIPTED] (8) | Like $ct[ACTIVE] but excludes unscripted physical objects. ||
| ::: | • Non-physical objects with an active script. (*) | $ct[PASSIVE]`|$ct[SCRIPTED] |
| ::: | • Physical objects with active scripts (*), or scripted physical objects that are moving. | $ct[ACTIVE]`|$ct[SCRIPTED] |
| $ct[AGENT]`|$ct[SCRIPTED] (9)\\ $ct[ACTIVE]`|$ct[SCRIPTED] (10)\\ $ct[AGENT]`|$ct[ACTIVE]`|$ct[SCRIPTED] (11)  | Same as $ct[SCRIPTED]. The $ct[AGENT] and $ct[ACTIVE] flags have no effect when combined with $ct[SCRIPTED]. ||
| $ct[PASSIVE]`|$ct[SCRIPTED] (12) | • Non-physical scripted objects (with scripts active or not). | $ct[PASSIVE]`|$ct[SCRIPTED] |
| ::: | • Physical scripted objects (with scripts active or not). | $ct[ACTIVE]`|$ct[SCRIPTED] |
| $ct[AGENT]`|$ct[PASSIVE]`|$ct[SCRIPTED] (13)\\ $ct[ACTIVE]`|$ct[PASSIVE]`|$ct[SCRIPTED] (14)\\ $ct[AGENT]`|$ct[ACTIVE]`|$ct[PASSIVE]`|$ct[SCRIPTED] (15) | Same as $ct[PASSIVE]`|$ct[SCRIPTED]. The $ct[AGENT] and $ct[ACTIVE] flags have no effect when combined with $ct[SCRIPTED]. ||
| 0 | Function silently fails (no event is triggered) ||

(*) Active script: a script with an active $lfn[llSetTimerEvent], $lfn[llSensorRepeat], $lfn[llListen] (no matter if the listen is disabled by $lfn[llListenControl]), $lfn[llTarget], $lfn[llRotTarget] and possibly more, or actively executing code at the time of scanning (e.g. in a tight loop, in $lfn[llSleep], etc.)

Although not tested, presumably $lct[AGENT_BY_USERNAME] works like $ct[AGENT] with respect to the types of objects detected.