There has been some confusion in the last year with regards to the difference between modular and semi-modular. Some people for example have been calling the Kilpatrick Phenol Semi-Modular. This however is not accurate, the Phenol is in fact a standalone modular synthesizer.
The difference between semi-modular and modular is that semi-modular has a hard-wired signal path that can be altered through patching, and fully modular instruments on the other hand requires patching. Modulars do not need separate units can be replaced. This is why the EMS Synthi and the Fenix are considered fully modular instruments. Semi-modular instruments of course are the Arp 2600, Korg MS-20, and more recently the Moog Mother-32.
The extent one can patch with a semi-modular synthesizers can vary greatly. The MS-20 for example lacks pre-filter/pre-mixer outputs for its oscillators. While the Arp 2600 allows for a great degree of patching between the various functions without such barriers. The Moog Mother-32 adds a feature in that it can be hooked directly into a modular system. The Buchla 208 was really what pushed out this concept. These instruments can even be experimental, many of the Ciat-Lonbarde instruments are also semi-modular, such as the Sidrax. These limitations though often allow for some benefits, specifically the ability to approach the instrument without patching. This can be helpful for those just learning synthesis to ease into modular functionality.
Modular synthesizers on the other hand come in two variety. One is modular systems, where you have cases or cabinets with modules you place modules or panels in. When you think of Moog, Buchla, and Eurorack, you are looking at a modular system. This is a common standard where functional modules can be taken out and added. There is another category of modular though, which is the stand alone. EMS pushed forward with this concept with it’s Synthi pin matrix synths. The Fenix, and later the Phenol bring this concept to the realm of patch cable synthesizers. These stand alone instruments do bring certain advantages, such as having all the necessary components for a voice, but still having all the advantages of signal path routing as a modular system. They also can bring increased portability, which can be an issue as modular systems grow in size.
Whether one is going with a fully modular synthesizer or a semi-modular, is often more about personal preference and budget.