First let me point out that the same conditional build tag
can be applied at either the topic level or the content level. It
sounds like you may be thinking that you have to create a tag as
for either the content or topic level.
Apply a tag at the topic level when you want the entire topic
excluded from a certain output. Do this by right-clicking on a
topic in your Project Manager, going to Apply Conditional Build
Tag, and selecting the tag.
Apply a tag at the content level (text, images) when you want
the topic to be included but want specific content to be excluded
from a certain output. Apply a tag at this level by selecting the
text and/or images that you want to apply the tag to,
right-clicking, going to Apply Conditional Build Tag, and picking
the tag.
Note that you wouldn't use a tag at the topic level on a
topic where you have used that same tag at the content level, and
vice versa.
Generally, the idea of build tags is to set an output to
exclude certain tags, but you can also set outputs to include tags
instead, which then excludes all other tags.
--Ben