The main page of any group holds the same functions that social wall does where you can start and hold conversations, ask questions, post photos, and share information or pictures with other users. Here you can post to the general public within the group or by gender for any gender specific questions or comments.
In addition to this, category posts are also published to this wall. A category is a subgroup without a web page. It is there mainly for the purpose of leading a group dialogue. You and other group members can only be invited to a category by the group administrator.
All of the messages that the category members exchange will be visible on the social wall to only them. The category's posts will be marked with the color the group administrator assigned to it while creating it.