An example link to specify which document should load in an iframe could be set up as follows:
<a href="your_page.html?ifrm=pg1">[link text here]</a>
The example links above this iframe are set up as you would to direct a user here from other locations. In-page links to load iframes would generally use a simpler method. The example in the download file demonstrates.
<iframe name="ifrm" id="ifrm" src="page.html"></iframe>