'Broadcasting' targets and 'tagging' targets are two different ways to communicate within a fleet. At first glance they appear to be much the same. But they operate just differently enough to make them more or less useful than each other depending on the situation.
Broadcasting labels a target with a number inside a circle both in space and on your overview and moves it to the top of your overview briefly. After a few seconds the broadcast disappears. You may broadcast multiple targets at once. Each target will be displayed in the fleet broadcast window until another broadcast is sent to replace it. Anyone in the fleet can broadcast a target.
Tagging does not put a marker on the target in space, does not move the target to the top of your overview, and does not broadcast the target in your fleet window. What it does do is allow a fleet commander to label multiple targets, up to twenty three, that stay tagged until they no longer exist in the sky. Tags come in two flavors, 0-9, and A-J plus X-Z. Some FCs use the numbers to indicate targets, and the letters to indicate triggers or tertiary targets. Tagging can be preferable when a commander is certain that multiple targets will be taken out in a given order. It has the added benefit of not needing to be rebroadcast in case someone in the fleet missed the original call. Only FCs, WCs, and SCs can apply tags.
Depending on the situation, either or both may be used. The primary reasons to use them is focusing fire. Both ON what you wish to hit and OFF what you do not wish to hit. An effective fleet kills quickly those that need killing. An effective fleet also does not kill what don't need killing.
By default, everyone can see broadcasts made within their squadron from the moment they start playing Eve.
In order to see the tags, however, you need to enable "TAGS" in your overview. This is actually quite simple. Open your Overview Settings window, click the Columns tab, check the box next to TAG. Done.
You can then drag the column to any position you want within the Overview Settings window. (My personal preference is in the second position.)
In order to tag a target you need to be a commander. (I'm not yet sure how the tag propagates throughout the fleet. If it's like broadcasts or not... I will update this post after I have done more research at which point I intend to create a fleet structure post.) Then right click the target and mouse over the word TAG which will expand a menu that includes numbers and letters. Choose one and move on to the next target. It's a pain, but the only way to do it right now.