That appears to be the definition of what license they wanted, not the actual license. The license body makes it unclear if I can use this in a closed-source commercial environment

On Thu, Mar 23, 2017 at 3:51 PM, <boost-users-request@lists.boost.org> wrote:
Message: 5
Date: Thu, 23 Mar 2017 17:54:04 +0100
From: Mateusz Loskot <mateusz@loskot.net>
To: boost-users@lists.boost.org
Subject: Re: [Boost-users] License questions
Message-ID:
        <CABUeae_a-cNCdw00fYN4Gyb_a7Gk_QOo7cARXNo3Lh8eXAAVtQ@mail.gmail.com>
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On 23 March 2017 at 17:38, Julian D. via Boost-users
<boost-users@lists.boost.org> wrote:
> I am making a confidential commercial closed-source program, and Boost's
> libraries seem useful for me. I can't tell if the Boost license allows me to
> include it in closed-source. Would anybody know?

http://www.boost.org/users/license.html#requirements

2nd bullet

Best regards,
--
Mateusz Loskot, http://mateusz.loskot.net