RH 7 - Context sensitive help?

Your i.e. " (i.e., a screen is displayed online and the user mouses-over the field name to get a help link or pop-up box appearing, telling the user what the field definition is)" appears to be the definition of a ToolTip....??

To create a tooltip, if the software application is a browser-based application, the programmer simply adds an ALT property to the object tag. That's inline with the website code--no calls to your help system at all. ALT content is a basic text string: no formatting, no links.
*As a help author, you might work with the developer to wordsmith the ToolTip text.

I suppose the webpage *could* alternately include some JS to recognize a MouseOver event and then call up a topic from your help system. If your help is in WebHelp format, they might just call the URL of the topic directly. You wouldn't do anything special in RH at all!! If your help is in CHM format, the programmer can call the topic either by topic file name (different syntax than calling a WebHelp topic) or you can assign a map ID number to each topic and the programmer would call the number instead. RH can help you set up the map ID file.

Welcome to the forum

Your help can be used to provide field level help. It's up to your developers how they call the topics with the field level help both in the sense of whether they use urls or one of the other methods and whether they just display the topic alone or in the tripane frame (toolbar, toc and topic).

Take a look at my site under Calling WebHelp.