Why -18? What's the reason to save 18db of headroom? I could understand -10 if you do audio post for television because that's what the network satellites are limited to, but I'm not familiar with the rationale for -18 in the digital domain.
The post stated that he was transfering to an analog console. In Pro Tools, -18db's is very close to analog 0. It is at this level which the analog gear can perform at its best. The -18db output only applies to those who intend to use analog gear during summing.