Amatixe/Unutixe (Tengwar)

Tom Bombadil #281

Could someone please explain what these things are, and wherefore they are needed in Tengwar? I neither know what kind of dots they describe nor what the "writing line" is.

PS. Are Amatixi all points and lines over the letters, like the diacritic marks for vowels? And are Unutixi all points and lines under the letters, like a pusta/putta, an upsilon, and a line to mark a double letter?

Elaran #282

Amatixi: Dots like the A and I tehtar.
Unutixi: Dots like the Y tehta in the Classical Mode for Quenya, and the silent E tehta in the General Mode of Tengwar for English.

These terms do not encompass any other tehtar, only the ones that are made with dots.