It is most likely no surprise to anyone who has engaged in numerous fleet combats that the visibility system doesn't work properly. The visibility patch does a number of things to try and fix this.
First, there was a bug where the first Tag_Values tag would get ignored in combat (the UniSpace and SystCost tags do not count towards the 'first tag ignored' rule). So, using ECM I as an example, it has the tags 'SystCost = 30, UniSpace = 15, DefTgtRg *= 1.12'. The first two are UniSpace and SystCost, so those are dropped. As far as the game is concerned, it only has one tag, 'DefTgtRg *= 1.12'. However, as was stated at the beginning of this paragraph, because of the bug, the first tag value is ignored. That means that ECM I does absolutely nothing in combat. In fact, none of the Sensor/ECM/ECCM/Cloak ship items do anything in combat.
Second, because of the way the formula for ship detection works, DefTgtRg (that is, decreasing he chance of being spotted by the enemy TF) was limited so that it could never provide more than a 1.33 benefit. In other words, if you used, say, ECM V, it should provide you with 'DefTgtRg *= 2.2'. However, the formula would reduce that to a benefit of only 1.33, or the equivalent of ECM III.
Third, the system was set up so that OffTgtRg and DefTgtRg had multiple functions. The TgtRg part of each of them stands for 'target range'. But not only did they affect targetting range, they also affected spotting range. Originally, there was another variable called SptRg (with OffSptRg and DefSptRg). Instead of keeping the two concepts separate, they were both merged into TgtRg. This made it difficult to design a system that worked for spotting, but didn't become overpowered for targetting (or vice versa).
Finally, there was the problem with OffTgtRg. In the previous paragraph it was stated that TgtRg affected targetting range. That isn't precisely true. While DefTgtRg functioned correctly, OffTgtRg did not, leading to severe problems with targetting.
The solution to all of these problems was multifacetted. TgtRg and SptRg were separated again. This means there are now four values to use instead of two:
OffTgtRg - Increases chance to hit.
DefTgtRg - Decreases chance of getting hit.
OffSptRg - Increases chance of spotting enemy TFs.
DefSptRg - Decreases chance of your TF being spotted.
There are two things to keep in mind about these tag values. For spotting range, the best value from the offensive (or the TF trying to spot) TF is compared against the worst value from the defensive (or the TF being spotted) TF. This means that only one ship needs to have a good OffSptRg (unless that one ship gets destroyed), but all ships need to have a good DefSptRg for cloaking to function properly.
Second, for targetting range, unlike spotting range, it works on a per ship basis, not a per TF basis (that is, the individual ship firing has its tag value used, as does the individual ship getting fired at). Targetting range also only affects beam weapons (this is unproven, but demonstrably likely), ships that aren't likely to engage in beam combat don't need offensive targetting, although they would still benefit from defensive targetting (as it will affect fighters and missiles that target your ship).
OffTgtRg was fixed so that it worked properly alongside DefTgtRg. The problem with ignoring the first tag value was also fixed. For this to have a proper effect on games, however, it is necessary that changes to TechTables.txt be done. Some modders have made the changes for you, however, if they have not, then you should use the TechTables.txt that is included with this patch (or, if comfortable modding, you can replace the required lines yourself).