Q. 1. What is the purpose of the "Altar crucifix?"

A. 1. An Altar Crucifix is placed upon the Altar, being the principal ornament of the Altar.

The crucifix placed upon the Altar is intended to serve as a reminder to the people in attendance and the celebrant of the believed nature of the Eucharist as the actual Body of Christ; the Altar itself symbolically represents Golgotha. It is for this reason that Roman Law decrees it necessary to have the crucifix upon the Altar whenever Mass is celebrated. Specifically, it is placed directly in between the Candlesticks in such a way that it is conveniently seen by the people. In some cases, to better fulfill this requirement, the crucifix is instead hung on the wall behind the Altar, so that when the priest is facing the congregation the crucifix is not obstructed. In some churches the crucifix is suspended mid-air via strong, nearly invisible metal cords, directly above the Altar itself.

While the crucifix is demanded to be upon or at least near the Altar at all times, during the period of time from the first Vespers of Passion Sunday to the unveiling of the cross on Good Friday it is expected to be covered with a violet veil, except for the High Mass on the Altar, when the veil is white, and Good Friday, when the veil is usually black. After Good Friday, until Holy Saturday it is necessary for all, including the bishop, the canons of the cathedral, and the celebrant to genuflect to the crucifix, which is in contrast to any other time of the year when the aforementioned are not required to genuflect.

[Source: https://en.wikipedia.org/wiki/Altar_crucifix]