In the link properties dialog box, there is a field called "Kind" which includes four kinds:
How are these kinds used?
Thank you Vladimir.
this field is used for filtering and selecting the links of certain kinds.
So called "soft" links can be added when you tried some option of organizing project work (build the road from the right to the left) and shall be quickly and easily removed when you decide to try different option. Filtering and removing soft links in the Links table easily solves the problem.
You may be interested to learn the impact of external links on the project schedule, etc.
Actually you can create User Defined Fields in the Links table and use any additional types of links if necessary. But these four link kinds are default and you better define that the link is discretionary if there are other ways to perform the job.