I haven't found information about these options:
is blocked by, blocks, is cloned by, is duplicated by, duplicates, relates to
An issue link is a simple pointer from one issue to another. The name of each link is supposed to tell you what the link is for.
They are mostly directional - in your list of six, you only actually have 4 link types there (assuming they are the standard ones - you can add your own link types and remove or rename all the system ones if you want).
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