Pound symbol in object type, in F#

I have come across this:

[<Interface>]
type ILogger =
    abstract Debug: string -> unit
    abstract Error: string -> unit 

[<Interface>] type ILog = abstract Logger: ILogger

module Log =
    let debug (env: #ILog) fmt = Printf.kprintf env.Logger.Debug fmt
    let error (env: #ILog) fmt = Printf.kprintf env.Logger.Error fmt

from that page: https://bartoszsypytkowski.com/dealing-with-complex-dependency-injection-in-f/

I was wondering what is the '#' symbol doing in front of the type: let debug (env: #ILog)...

the page's explanation is this:

The secret is in #ILog signature, which means that our environment can be any generic type implementing ILog interface.

but I tried to find documentation on this and my google search wasn't successful. Where can I read more about it?

my understanding is that you can pass objects that implement a specific interface and the functions that use the # will auto cast the object to the interface and use it.

But then why do they implement it as 2 interfaces instead of a single one here?