What is this blue holder in the picture above? Why is it not in the manual?

Welcome to the forum!

You've made Flowed Text.  It's explained here in the manual:  http://tavmjong.free.fr/INKSCAPE/MANUAL/html/Text-Creating.html#Text-Enter-Flow

Unless you need flowed text for a reason, it's best to avoid using it.  It can't be seen or used outside of Inkscape.

To make regular text, you should click once on the canvas.  When you see the cursor flashing, you can start typing.  Or you can convert flowed text to regular with Text menu > Convert to Text.

it is there by default.

I just click "T" then click the image field, not drag.

And we dont see here a blue rectangle, aren't we? I tried dragging it on a multiline text, and there two vertical lines appear. The right one is really dragged, but the text beyond the line just disappears; I mean, including all the folowing "visible" lines.

Looks like a bug, and orphaned, not working, code from that "flowed" text feature.

If I were you, I'd un flow it (there's an option), or possibly copy the text and place it in a regular text box instead of flowed text.  There's issues in working with flowed text and I would not be surprised if it doesn't disappear one day (my thoughts), but I simply avoid using it.

Flowed text will probably stay in Inkscape, even though it won't be useful in browsers and some other renderers. 

It does seem to work in PDFs:

japanpersonals

I just click "T" then click the image field, not drag.

I suggest not to use the Text and Font dialog.  Everything you need, at least for now (with the current stable version) is on the control bar (the first bar of icons above the horizontal ruler, which changes with each different tool that you enable).  In the future, that dialog will apparently have some other text tools.  But for now, I just find it hard to use and confusing.

japanpersonals

And we dont see here a blue rectangle, aren't we?

Yes, I see what you mean.  Is there any chance that's a node (cusp/corner node) from a nearby path?  It's very small, localized screenshot and we aren't sure what's outside it.

japanpersonals

I tried dragging it on a multiline text, and there two vertical lines appear. The right one is really dragged, but the text beyond the line just disappears; I mean, including all the folowing "visible" lines.

I'm not sure what you are decribing there.  Can you make another screenshot to show?  Or you are welcome to share your SVG file with us, and then we don't have to guess, we can know for sure.

If you drag the Text tool on text that has already been typed, it should maybe highlight it or maybe move it.  But I'm not sure what you're describing about multiline text.

I'll make a screenshot....  Actually, I think part of attachment feature in the forum is broken right now.  But if you click on blue "Attachments" button you can see my screenshots.

rttft.png shows where I made the text box, and I'm ready to start typing flowed text.  You can barely notice the cursor in the top-left corner.

tft.png shows where I started typing some flowed text.  And mft.png shows where the box fits the text closely.  I do agree that your first screenshot does not look like flowed text after all.

That screenshot in the OP is from the beta version. It is not released yet.

The manual is always a couple versions behind - and it is not for unreleased versions.

You can find information about new features in the beta version in the release notes draft:

http://wiki.inkscape.org/wiki/index.php/Release_notes/1.0

Direct link: http://wiki.inkscape.org/wiki/index.php/Release_notes/1.0#Browser-compatible_flowed_text

Ah, yes, Maren, it is 1b1. I had an impression that the last stabile version was tha same, but I was probably mistaken.

Brynn, of course, top bar is useful, but it does not change the current tool(input mode). There are no other objects in the screenshot, just blank space - the screenshot is purely for compact and exact illustration.

Btw, now I see a blue frame, when making longer text, and scaling in/out. But it is different from the controller, see below:

(dragging the controller left)

I have no idea why the controller affects only one side of the flow text, and does not offer any other modification, of it is the flow text you have described.

Quote from the link I gave above... :
 

SVG 2 text with 'inline-size' property: This is the new "Column mode" of the text tool, that can be created by clicking on the canvas, typing, and then dragging the diamond-shaped handle at the end of the text to determine the width of the text. It creates flowed text columns that without a predetermined height. These texts include an SVG 1.1 fallback, too. To convert the text back to SVG 1.1 text, Ctrl-click on the diamond (this will result in all the text being placed on a single line).

With your last screenshots, Someone fromjapan, those are features apparently which I have not seen yet.  Did not know which version you were using until now.