I don't know what that parameter is, but I do know that when dealing with tangents, AstuteGraphics has a plug-in for that called SubScribe. Their plug-ins are free for 7 days I believe, so you can give it a try and see if it resolves your problem.
Are you using this plug-in? If so, does it have many advantages?
It has a lot of advantages. Crating tandents to two curves is a breeze with that plugin. Also Subscribe is licensed as a package with several other plugins. That whole package offers a lot of functionality. But you will have to look closely and check if you need them in that extent that it provides the added value to your individual workflow.
As a beginners I may not need this now but in the future I may need this.
D is the length and the angle is the angle of the line. 0° would be horizontal to the right.
To get a perpendicular line to your radius, duplicate it and rotate by 90° around one of its endpoints
Please confirm whether I indicated correct D location & angle location. In second screenshot, what is meant by On and Line Extension ?
THose are annotations that indicate where you have clicked and what you are transforming.
Without knowing the angle of that radius line, nobody can tell whether the new line has the correct angle.