DVT SystemVerilog IDE User Guide
Rev. 19.1.12, 19 April 2019
Some of the source files in your project might not be part of the full compilation (they are not referenced from the .dvt/default.build). For example code fragments which have not been integrated yet, or components which are part of a different build configuration than the active one (see Multiple .build Files). For convenience, all such files are called non-top files.
You can easily identify non-top files as they are missing the blue bullet decoration.
In order to provide advanced functionality when editing non-top files, DVT compiles them ad-hoc when opened, in the global scope. As a result, new problems might be reported and DVT's internal database is updated, which is reflected in the contents of views, autocomplete, design and class hierarchies etc.
When you are done editing a non-top-file and close the editor, all the content contributed by that file is removed: problem markers, types and API, etc.
Note: Verilog non-top-files are compiled as if part of a library called "non-top-files".
Note: The message of problems reported in non-top files are prefixed with an underscore. This is an indication that problems may be caused by compiling a file ad-hoc out of context.