Reference ________ requires checking as it has been updated.
This warning may appear when re-tagging a document - that is, re-running the automatic Tag x-refs function again on a document which already contains references. The warning will appear if you have made changes to the text of your document in a way which causes the meaning and implicit target of certain references to change. The warning alerts you to references which originally referred to a particular provision of a document, but which now (after re-tagging) refer to a different document, usually because something else has changed nearby which affects the context in which that reference appears.
For example, a provision of a Bill which makes amendments to enacted legislation will often introduce those amendments by stating that “the [Act name] is amended as follows”. The other subsections or paragraphs which follow will contain references to the provisions in that Act without explicitly stating after each reference that they are references to the same Act. Automatically tagging such a provision in Lawmaker will correctly identify that the references in the other subsections/paragraphs are to provisions in the Act mentioned in the introductory text. If you subsequently change the text which introduces the Act so that it instead refers to a different Act, and re-tag the provision, the references in the other subsections will be flagged for checking. The screenshots below show an example of this:

After first automatic tagging

After editing the introductory reference in paragraph 11 (so that it reads “the Companies Act 2006”) and re-tagging
The purpose of this document check is to alert you that any automatically tagged references which have been updated due to a change of context and ought to be checked.
To check them, you can double-click on each one to bring up the floating toolbar, then click Go to ref to go to the target of the reference.
If the reference still appears correct to you, go to the Document Checks pane on the right-hand side of the Editor screen, locate the entry relating to that reference (clicking on each will highlight the relevant reference in the Editor), and click the ‘Confirm reference is valid’ quick-fix option:

The document check and quick-fix options for a “Check” reference
Alternatively, double-click on the text of the reference to bring up the floating toolbar, select Edit ref, and in the modal scroll down and change the Status from ‘Check' to ‘Valid’. Then click Save to save the changed status.

The Status field in the Edit ref modal
If the reference target appears incorrect to you, it is advisable to re-read the provision in which the reference appears and consider why the target of the reference has changed - there may be a good reason why the target has been updated. If you still aren’t sure why the target is incorrect, you may then choose to delete the reference and re-create it manually (for example if it is an internal reference which has been incorrectly tagged as an external reference). Alternatively, you can right-click on the reference and choose ‘Ignore reference’ to remove the link and also exclude the reference from further consideration by any re-tagging operations.