One of the areas that is often confusing to new users of MODO are the various selection modes, causing a variety of seemingly baffling issues. It's a very important concept to understand (which is why it is mentioned so often here in these docs). When in their respective selection modes, only selections of that type may be made, keeping in mind that when nothing specific is selected, then everything in a layer is selected (another important concept!).
The first three modes 'Vertices', 'Edges' or 'Polygons' are called the component modes. Users can press any of the interface buttons to enter these modes, and press the space bar to toggle through them sequentially. The next mode is called 'Item' or 'Items' mode, users can enter this mode by LMB+clicking the button directly within the interface. It can also be handy for users to know the mode keyboard shortcuts, pressing '1', '2' or '3' respectively enter 'Vertices', 'Edges' or 'Polygons' modes. Pressing '5' will enter 'Items' mode. These aren't the only modes, but they are the most important.
Users can transform their geometry while in any of these modes (transform as in move, scale and rotate). When applying these kinds of changes, the big difference between what happens with each lies in the layers center point. When in a component mode, applying any type of transform, users are editing the raw geometry that makes up any model; these are called component level transforms. Regardless of the position of the geometry in the mesh item layer, the center position for that item in the properties panel will always remains at the world origin- 0,0,0.
Now when entering 'Items' mode, and applying a transform, users are applying what is called an item level transform. Moving the geometry in 3D space, the values in the items properties panel are now changing from something other than 0,0,0, moving them away from the world origin. Users can no longer affect individual vertices (or edges, or polygons) in 'Items' mode, as the layer as a whole is being modified. This is how objects are animated (and positioned for layout) in MODO, by applying item level transforms. (For more on the Mesh Items properties panel, please reference that page of the documentation) It can be said that modifications made in the Vertex, Edge and Polygon modes happen around the layers center point, while transforms applied while in Item mode move the entire item including its center point.
It can become confusing as the same tools for move, scale and rotate are used for transforming items in component and item mode, but if you keep an eye out for which mode you are in (usually based on what is being selected), it will soon become second nature. It should be noted that the component modes only apply specifically to mesh item layers, other layers such as lights, texture locators, replicators or cameras can only be edited while in Items mode.
Other Modes
While on the subject, the other selection modes not mentioned above are the 'Center', 'Pivot' and 'Material' modes. While in the prior two types, users can change the position of a 'Center' or 'Pivot' relative to the geometry or item, essentially modifying the layers origin position. Because of Action Centers, the positions of 'Centers' and 'Pivots' aren't all that important until an item needs to be animated, then it becomes very important because the Center represents where the animation transforms originate from. Rotating a tire when the Center is positioned below the tire would not produce the desired results. But when the tire is already in the proper position relative to a car body, its much easier to just transform the 'Center' itself moving it to the proper position at the axle so the wheel can spin smoothly. The 'Animate' section has a number of setup tools that can make it much easier to modify geometry and Centers relative to each other. In order to manipulate a Center or a Pivot, users must first select the item while in the particular mode, by LMB+clicking on the Center or Pivot icons in the 3D viewport and then transforming them. This extra step makes it more difficult to accidentally change their respective positions.
The final selection mode is 'Material' mode. While users can select and transform geometry based on Material assignments (actually tags), it is mostly useful for easily selecting geometry based on particular tag types. Users can change precisely what type of tag the mode specifically selects by adjusting the settings in the "Select > Polygon Tag Type" menu. In Materials mode (the default), the selection function will not only highlight all the geometry in the scene with a specific material tag, but it will highlight the associated Shader Tree layers making it easier to pinpoint a particular layer in a complex tree. In the 'Part' and Smoothing Group' options, either Part tags or Smoothing Group assignments are selected respectively.
MODO provides users the flexibility to work in the way that is most comfortable. Take for instance these two examples, the user that wants to Radial Sweep a tire model around the origin and position it in the wheel well, or the user that wants to Radial Sweep the tire in position and adjust the center point after the fact; either way will work. Users should always be aware of an objects Center and Pivot position, so when issues arise, such as when working with Replicators, it will be easy to spot the fix.
