From:
https://math.hws.edu/eck/cs324/s01/pov-doc/pov171.htm

Operator Promotion
You may use a lone float expression to define a vector whose components are all
the same. POV-Ray knows when it needs a vector of a particular type and will
promote a float into a vector if need be. For example the POV-Ray scale
statement requires a three component vector. If you specify scale 5 then POV-Ray
interprets this as scale <5,5,5> which means you want to scale by 5 in every
direction.

Versions of POV-Ray prior to 3.0 only allowed such use of a float as a vector in
various limited places such as scale and turbulence. However you may now use
this trick anywhere. For example...

 box{0,1}  // Same as box{<0,0,0>,<1,1,1>}

 sphere{0,1} // Same as sphere{<0,0,0>,1}
When promoting a float into a vector of 2, 3, 4 or 5 components, all components
are set to the float value, however when promoting a vector of a lower number of
components into a higher order vector, all remaining components are set to zero.
For example if POV-Ray expects a 4D vector and you specify 9 the result is
<9,9,9,9> but if you specify <7,6> the result is <7,6,0,0>.

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