What´s the difference between Text and "Place Note-- Callout" ?

While I can't answer this completely...

A text element is strictly that. A piece of text. It has a text style (or none, if folks go that way) but no other associated information.

A Note, of any kind is a different element entirely. The composition of a note is somewhat obscure - it has a text style and a dimension style, but as far as I can tell is neither a text nor a dimension. The leader may be an associated dimension element (I forget) but the note is it's own thing.

Notes have traditionally been difficult to update because of this. Updating either the text style of the note or the dimension style of the note often has unexpected results.

I do not know if it is possible to add a leader to a note that was not created with one. I do know that it is possible to create and associate an additional leader to an existing note that does already have a leader: https://communities.bentley.com/products/microstation/f/microstation-forum/211590/ss10-add-extra-leaders-to-existing-note

I consider Notes a hybrid element. If you hover over the terminator or leader line they display Dimension. Hovering over the Text or in-line leader they identify as Note. The leader & in-line leader are affected by the Dimension Styles settings, but the text is affected by the Text Styles settings.

There are issues with modifying the notes with leaders. The most annoying for me is when you change the level of a note with leader because the in-line leader has an association to both the note & the dimension elements. Our leaders are normally weight 1, but the text, depending on existing or proposed, is weight 1, 2 or 3. Using the Change Element Attributes tool and switching the level (style, color & weight are bylevel except the leader) and selecting only the text, the in-line leader becomes weight bylevel. Then you select the leader (setting the weight to 1 in the Change Attributes dialog). Now the in-line leader becomes weight 1.

Prior to the the new Element Information tool you could select the note with leader and the legacy Analyze Element tool would show a breakdown of the individual elements of the note. You were able to select just the in-line leader and modify its symbology. With the new Element Properties the note shows as a single element and you cannot modify the the individual elements that make up the note. The legacy Analyze Element dialog is still available if needed.

The only way I know to add a leader to an existing note that was originally placed without one is to use the Edit Text tool to select the text. While the text is in the editor switch to the Place Note tool. You still need to define the terminator anchor and select the location of the text in the drawing.

Don't even get me started on the frustration of editing the in-line leader length. That's a process and it has changed from how it worked in v8i. 

I consider Notes a hybrid element. If you hover over the terminator or leader line they display Dimension. Hovering over the Text or in-line leader they identify as Note. The leader & in-line leader are affected by the Dimension Styles settings, but the text is affected by the Text Styles settings.

There are issues with modifying the notes with leaders. The most annoying for me is when you change the level of a note with leader because the in-line leader has an association to both the note & the dimension elements. Our leaders are normally weight 1, but the text, depending on existing or proposed, is weight 1, 2 or 3. Using the Change Element Attributes tool and switching the level (style, color & weight are bylevel except the leader) and selecting only the text, the in-line leader becomes weight bylevel. Then you select the leader (setting the weight to 1 in the Change Attributes dialog). Now the in-line leader becomes weight 1.

Prior to the the new Element Information tool you could select the note with leader and the legacy Analyze Element tool would show a breakdown of the individual elements of the note. You were able to select just the in-line leader and modify its symbology. With the new Element Properties the note shows as a single element and you cannot modify the the individual elements that make up the note. The legacy Analyze Element dialog is still available if needed.

The only way I know to add a leader to an existing note that was originally placed without one is to use the Edit Text tool to select the text. While the text is in the editor switch to the Place Note tool. You still need to define the terminator anchor and select the location of the text in the drawing.

Don't even get me started on the frustration of editing the in-line leader length. That's a process and it has changed from how it worked in v8i.