Fixed Do
Do always designates the same note, independent of the key — the normal case in the Romance-language area.
What it means
In fixed Do, the solmization syllables designate absolute pitches: do is always C, re always D, mi always E, regardless of the key. The syllables are thus simply note names and take the place of the letters that are usual in the German and English-speaking areas.
Musical use
In the Romance-language area — Italy, France, Spain, Portugal, and large parts of Latin America — this is the normal case: there the syllables are the only note names in common use, and a key designation accordingly reads sol maggiore for G major. In this, si corresponds to the German H, that is, B.
For ear training, fixed Do has a different character from movable Do. It ties the syllables to absolute pitches and thus meets the needs of absolute hearing, but says nothing about the function of a note within the key: in G major sol is the root, in C major the fifth, and the syllable does not distinguish this. Whoever wants to learn to hear degrees is better served with movable Do; whoever is trained in a country in which the syllables are note names learns them that way anyway.
Altered notes are designated by appended words for raising and lowering, corresponding to the accidentals.
Fixed Do is taught in France, Italy, Spain, and many countries with a Romance language, where the syllables are at the same time the note names. It trains absolute note awareness and the direct connection of syllable and notation. A disadvantage is that the syllables say nothing about the position of the note in the key and chromatic notes have no syllable of their own. Here lies the basic difference from movable Do, which makes relationships between the degrees audible. Many training institutions therefore combine both procedures.
Performance practice
In mixed groups, settle which system is being used — the same syllable means something different in the two, and misunderstandings are stubborn. If you work with Romance-language editions, note the keys once in German spelling. And for your own ear training choose one system and stay with it.