Have you been able to find the answer to this question yet? The convertible preferred shares benefit from common price increase because an investor could convert the preferred shares to common shares at the pre-specified conversion rate. If I remember from the reading correctly, these preferred shares receive a smaller dividend for that advantage.
For issuance, ATM would mean that the company would sell them at the rate that is available in the market. Via placement would indicate, to me, that the issuer negotiates an agreement and sells the shares to a private entity at a specified price.