Chapter V of PCS begins with an introduction to viaticum (175-188). Sections 189-211 give the rubrics and texts for the two forms of viaticum.
175. This chapter contains a rite for viaticum within Mass and a rite for viaticum outside Mass. The celebration of the eucharist as viaticum, food for the passage through death to eternal life, is the sacrament proper to the dying Christian. It is the completion and crown of the Christian life on this earth, signifying that the Christian follows the Lord to eternal glory and the banquet of the heavenly kingdom.
For non-Catholic readers (and probably some Catholics not fully informed) there is a very important distinction made in the sacramental theology and pastoral practice of a Christian’s final illness and dying process.
The sacrament of the anointing of the sick should be celebrated at the beginning of a serious illness, Viaticum, celebrated when death is close, will then be better understood as ther last sacrament of Christian life.
So we see anointing strengthens the believer at the onset of illness. Viaticum marks the end of the final struggle. In between, the Christian is regularly nourished by Mass and the other sacraments.
My impression is that we have made some progress on this principle since Vatican II. I wonder and worry that this important theological distinction is lost on advocates of the traditional Latin Mass.