In the days of MetaFrame XP, updates were simple. About once a year, Citrix would release a "Service Pack" and a "Feature Release" on the same day. In fact, the Service Pack and Feature Release packages were 100% identical. The only difference was that if your Subscription Advantage was current, when you installed the Service Pack you could "unlock" new features that comprised the Feature Release. If you did not have a license for the new Feature Release, then you would just install the Service Pack and leave it at that.
That technique worked well for years until Citrix decided to change it. For some reason around the time of Presentation Server 3.0, Citrix decided that they would do away with Feature Releases and instead just release versions of their products. (3.0, 3.1, etc.) This was first seen with Presentation Server 4.0. As you can see from the table, Citrix went directly from PS 3 to PS 4 without any Feature Releases. So people assumed this was the "new system" at work. But what Citrix didn't really tell anyone was that nothing really had changed. Citrix released Service Pack 1 for PS 3 at the exact same time that they released PS 4. And guess what? The SP1 code for PS 3 was the 100% same identical code for PS 4! This led to a lot of "accidental" migrations to PS 4. Most customers would never upgrade from PS 3 to PS 4 without major testing. But simply installing a Service Pack? That was something they would do more readily. So people installed SP 1 into their PS 3 farms and decided that they'd wait before going to PS 4. But when they did decide to go to PS 4, then didn't have to install any new code! All they had to do was flip a switch in the management console, and voila! They were on PS 4!
PS 4 didn't have any more Service Packs though. Around this time Citrix decided that they would release "Hotfix Rollup Packs," or "HRPs," instead of Service Packs. Again no one really understands the logic behind this. Some people thought it's because "HRP" sounds less dangerous than "Service Pack," so maybe people will be more inclined to want to install the HRPs. As its name implies, people assumed that an HRP was simply a "rollup pack" that bundled many hotfixes into a single MSI. While this is true, what Citrix failed to tell people is that the rollup packs ALSO added new features to the products!
To this day, that is one of the biggest complaints that people have about Citrix. The problem with adding new features to a "rollup pack" is that, sometimes new features don't work exactly right, which means you have to patch or re-release the new features. Ordinarily this would not be a problem (and would even be expected), but when your new features are released in a "hotfix rollup pack," how do you patch them? Now you have to re-release or patch the rollup patch!
This also means that people didn't trust the integrity of the rollup packs. Citrix support tells people that rollup packs should be installed as soon as they're released, but Citrix has had to re-release the rollup packs far too many times for people to fall for that. So now Citrix releases a hotfix rollup pack, and most customers wait a few weeks for the dust to settle before actually installing them into their own environments.