The four different Icom earpiece connectors connect the earpiece in question to four different types of two-way radio or, at least, four different types of connector.
Generally, it is good advice to make absolutely sure that your chosen earpiece can connect with the radio you intend to use. Not all radios use the same fittings, which is why there are so many different types of connector available.
A straight pin connector, for example, works best when used on marine radios, but is also compatible with Maxon and Vertex radios. Another thing to take into account is the activity that the earpiece is going to be used for. If the user is going to be standing still for prolonged periods of time, then he/she may require a different type to the one that may be preferable if the user is going to be engaging in strenuous exercise.
Two-way radios are used in a great many distinct environments, including shopping centers, building sites, roadway maintenance, fleet and transport operations, shipping, factory floor work, security, healthcare, private transport and much more besides. Because of this, there are many different types of earpiece connectors.
The four available Icom earpiece connectors indicate the jobs that the compatible two-way radios were likely designed for. This does not necessarily mean that an Icom radio would be unsuitable for other work, just that it is probably particularly suited to a certain type of job.
The different connectors also have distinct methods of working, even though their basic external operations are usually indistinguishable from one another. Icom have gone to great lengths to make their earpieces among the best in the world. One way they have done this is by allowing the earpiece connectors for icom to be tailor made for specific tasks.