A comment attaching line is a dashed line that is used to attach a comment (note) to a model element in a diagram. The comment (note) attaching line is not a UML relationship and has no semantics beyond signifying that the contents of a comment apply to the model element it is attached to. A comment may be attached to any number of model elements or may exist in isolation and by placement in proximity to one or more model elements signify that it applies to those elements.
The comment connector line is semantically weak and simply serves as a device to relate a comment to a model element. A comment that has no connection to any elements can be ambiguous as a reader may incorrectly assume which elements it applies to. Attaching the comment to one or more elements will add clarity to you diagrams and model.
A note attacher may be used with any model elements including UML relationships.
A note attacher may be used with any model elements including elements like association relationships, activities and decisions. Typically a separate line is drawn between a note and the model elements it is attached to.
A comment is related to a comment attaching line because it can be attached to a model element by a comment attaching line. If the comment attaching line is not used the comment can stand freely in a diagram.