Hello Alexis,
It works for me, although does not mention planes specifically, but lumps them as "nonatomic models":

<https://rbvi.ucsf.edu/chimerax/docs/user/commands/atomspec.html#othermodels>

"Nonatomic models such as nonmolecular surfaces or maps (volume models) can be specified in a more limited set of ways, namely:
        • hierarchical specifiers – model number
        • built-in classifications – sel (the current selection, which could include nonatomic models)"

Examples:

open 2gbp
define plane ligand
color #1.2 red

(or if you want to select it by model number:)

select #1.2

(or you can select in the graphics window with Ctrl-click, then refer to it as "sel")

color sel cyan

I hope this helps,
Elaine
-----
Elaine C. Meng, Ph.D.                       
UCSF Chimera(X) team
Department of Pharmaceutical Chemistry
University of California, San Francisco

> On Nov 16, 2023, at 2:52 PM, Alexis Rohou via ChimeraX-users <chimerax-users@cgl.ucsf.edu> wrote:
>
> Hello,
>
> I'm playing around with the define axis command, and noticed that in the documentation the link to "plane-spec" does not lead to anything that defines how to specify a plane (help:user/commands/atomspec.html#othermodels). Or at least, I can't see it. The link to "point-spec" does take me to an informative page (help:user/commands/pointspec.html#point).
>
> Is the syntax for specifying a plane defined elsewhere?
>
> This is with 1.6.1.
>
> Cheers,
> Alexis
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