As you might expect from the name, the AEC Trim command's function is similar to the TRIM command, but the order in which you do things is different. Rather than selecting cutting edges, and then items to be cut, as in the TRIM command, when using AEC Trim command, you select the items to be trimmed first, then define a cutting edge by selecting two points on the screen or by pressing ENTER and then selecting an on-screen line segment that is then used to define an "infinite" line that is used as the cutting edge. Linework to be trimmed does not have to intersect the selected line segment. In addition to lines and polyline segments, you can use lines from ACA objects, such as Wall Component Boundary lines or AEC Polygon lines. While it will allow you to select an arc, circle or polyline arc segment, in my experience it has deleted the entire item to be trimmed when a curved segment is chosen.
There are a few other differences. One major difference is you can trim block references. No change is made to the original block definition and any untrimmed block references remain unchanged. Instead, the trimmed block references are replaced with new "anonymous" block definitions where the defining linework has been trimmed at the cut line, where possible, as shown in the following images.
Two block references, on left side, selected for trimming.
Line in drawing selected as boundary edge for trim.
Side to trim selected.
Result of block reference trimming.
Note that the linework on the side to be trimmed highlights in red (ACA 2015 only), to help you visualize the trim results. Note also that in the fourth image, the layer of the trim line has been frozen, and that there is a line along the trim line location in each of the trimmed blocks. This is because the blocks contained two closed objects: a circle (magenta) and a pentagonal polyline (blue). Unlike the TRIM command, when trimmable closed objects are trimmed using the AEC Trim tool, the result remains a closed object.
AecLineworkTrim on Closed Polyline, Circle and AEC Polygon.
AecLineworkTrim Results on Closed Polyline, Circle and AEC Polygon.
Trimming the closed polygon (top pentagon) results in a closed polygon, although the closing line does not pick up the segment width of the original segments, if any. The TRIM command would have left an open polyline. Trimming the circle results in a closed polyline, rather than the arc that the TRIM command would have left. Trimming the AEC Polygon results in an AEC Polygon, rather than the open polyline that the TRIM command produces.
NOTE: There appears to be a bug in the 2015 version that results in the wrong side of AEC Polygons being kept after the trim, as can be seen in the image above. The command works properly in the 2014 release (but you will not get the red in-canvas preview of the linework that should be deleted in 2014).
Elipses and elipse arcs cannot be trimmed using the AEC Trim tool, and will not be included in the selection set. Walls, Doors, Windows, Door/Window Assemblies and Multi-View Blocks also will not be trimmed, even though they can be selected and, if the trim line crosses them, will show the red highlighting implying that they will be trimmed. There are likely other non-trimmable items; I have not tried to test every single object type with the command. I believe the intent of these tools was to work with the items generated by the Detail Components feature, so other objects will not necessarily play nicely.
If an untrimmable object is nested within a block definition, it will not be trimmed if you trim an instance of that block. Any trimmable items will, however, be trimmed.
You can also access the command from the right-click context menu, which can be handy if you want to select multiple objects first, before choosing the command.