I wanted to jump in here because in this discussion, a lot of people have said or implied things like (paraphrasing) "top to bottom in << // // // >> should correspond top to bottom in the score", and suggesting naming conventions based on this.
These thoughts, while well-intentioned, are bad since they are misleading for this important reason:
!!! Top to bottom on the staff has precisely nothing to do with any conventions of << // // // >> !!!
Top to bottom on the staff has ONLY to do with the relative pitches. For example, whenever an Alto part goes lower than the Soprano in an SATB arrangment, then the staff order (during that voice crossing) from top to bottom is ASTB, not SATB. So, staff order is not necessaritly a fixed property, even within a single << // // // >>. Also, unlike stem direction, the top-to-bottom score order cannot be overridden, so it is not even a property of this construct.
Therefore, in terms of making it clear to someone trying to use this for the first time (and thereafter remembered), it seems to me to make sense to try to use names that make it clear that the two main visual dimensions are:
1. Stem direction
2. Indentation level
Since these are what the << // // // >> construct actually does to the music you enter.
So, I like the suggestion of \voiceUpOne \voiceDownOne \voiceUpTwo \voiceDownTwo except that the "One" and "Two" don't tell you anything useful about what your voice will look like graphically, musically or in code order.
Also, the only voice whose number matches its order is "UpOne". The second voice is "DownOne"? Aren't we back where we started with misnumbering?
In terms of this approach, I think it would be clearer as:
\voiceUpOne
\voiceDownTwo
\voiceUpThree
\voiceDownFour
\voiceUpFive
\voiceDownSix
However, I most liked the spirit behind the previous suggestion for something like \voiceUp \voiceDown \voiceUpInner \voiceDownInner, although I don't like the word Inner since it suggests something about the relative pitch order of the voices, which (if you bought into my rant above) is bad.
So, I would suggest the term "Indent" or something else that indicated that the voice is to be treated visually as such:
\voiceUp
\voiceDown
\voiceUpIndent
\voiceDownIndent
\voiceUpIndentMore
\voiceDownIndentMore
Or, if you prefer Teletubbies vernacular, for these last two: