it depends on your requirement. search engines treat sub-domains as a different domain so you need to build backlinks and trust. on the other hand, sub-directory (child pages) have an advantage as they automatically inherit the authority from the main domain.

if you are starting a completely different thing whose content will be quite comprehensive and different but want it to relate to your main domain then start it on a sub-domain. on the other hand, if the content is not too large and it related to your main domain then use sub-directory

Sorta as Vicky suggested, both methods have pros and cons but can be equally as optimizable and condusive to high rankings.

Personally, SEO-wise, I prefer the sub-domain (abc.mysite.com) option; provided the whole package is well cross-linked together to gel it all into one big bunch of webpages each having its own unique but related content. I like sub-domains too because it makes it easier to group webpages that belong together (directories can work well for as well [mysite.com/web-pages-a/] but lose out in gaining a slight reward for having a keyword within the domain name; as Vicky pointed out, the search engine will consider it a separate domain but with effective and relatively heavy crosslinking the searchengine will eventually associate it with the root domain and the subsequent sister sub-domains).

The other option of having unique webpages within the root directory is a good one too, it also allow for a few important SEO advantages; including the ability to logically name the important webpages (mysite.com/web-page-a.htm, mysite.com/web-page-b.htm).

Of course, ideally, you could accomplish this using a little bit of both methods ie: abc.mysite.com/web-page-a.htm and don't hesitate to go as far as to offer the search engine something as diverse as abc.mysite.com/importantsection/web-page-a.htm (although the name of the directory [importantsection] will not provide any rewards, at this time, that I'm aware of, but can be a good sub-level divider).

thanks for guiding.

It's important to make the distinction between a sub-domain and a 'child page', or more accurately referred to as a URI.

Sub-domains are actually websites, whereas 'child pages' are pages within the domain. So, you could actually have the following:

www.mysite.com (a website)
mysite.com (a website, same as www.mysite.com)
www.mysite.com/some_page.html (child page)

abc.mysite.com (a website, separate from www.mysite.com)
abc.mysite.com/some_page.html (child page)

To answer your question directly, search engines have no preference. I would however suggest you select a URL structure and stick with it. The last thing you want is to try and game the SE's with multiple sub-domain/child page combinations.

Hope this helps!