The address for the list is the same for all topics. The topic for a message is designated in the subject line of a post. Otherwise the computer will not recognize it. Blank spaces, commas, hyphens, or other punctuation marks will not work. There are VERY FEW occasions when it is necessary to post to all sections.
Someone wants to reply to a message from any section of the list.
The reply is meant for the same section where the original message was posted. The reply will be posted in the same section as the original message. Someone wants to reply to a message, but to change the section. An autistic person or a cousin may see a post in the Parents' Auxiliary and want to respond with some perseverative word play, or number play, or other autistic-style fun. Or a parent may see a post in an AC-oriented section and want to respond with comments about parenting issues.