Parts Language
Our system used to not use parts language for our system, but now that our system is more complex and its functions are better understood, we find it more useful to talk about having "parts" now. However, we use this word somewhat more broadly and fluidly than other systems might, as well as sometimes more like the way a singlet talks about having a "part" of themself.
Usually, when we use "part" to refer to the system, we use it to refer to a large group in the system who all have something in common, are in the same amnesia group/sidesystem, split for the same reason, etc.
For example, we talk about West Voidelia/Blue's Supercluster as a part, as well as Upwards Spiral (a subsystem whose core part is no longer in the system has himself). However, we also talk about "the part of the system that was abused online" as a way of condensing that aspect of the collective self into one being, because not all members of the system were abused online.
We sometimes use "part" when we don't know or don't want to specify if we're talking about one headmate or more than one headmate. For example, there are times where it's clear that someone in the system did something in the innerworld history, but we're unsure if it was done by one headmate, a few headmates, a subsystem, etc. Therefore, we might say something like "the part responsible for getting us into [XYZ situation]" to refer to an unknown segment of the system who did something.
Individual headmates in the system also often refer to having "parts" of themselves sometimes. Sometimes this is used for situations where someone feels they have some kind of median subsystem and is speaking on behalf of those members of the system.
However, singlets also talk about having "parts" of themselves, like when they're torn between multiple options or they do something uncharacteristic sometimes, and we often use "part" for ourselves as individual headmates in this sense.
Individual headmates also refer to "parts of themselves" in situations where they're unsure if something they remember or think they did in the past was done by the current version of themself or if it was done by a past version of themself, or a headmate that split off of them and may no longer be in the system anymore. If it's the latter, then the reasons the current headmate did whatever they're talking about may no longer apply to them, but it was still them who did the thing. Therefore, they may talk about "the part of me who did XYZ".
While we do sometimes use "part" as synonymous with "headmate" in a straightforward way, this is much less common than any of the above uses when we're talking about our system. We basically do mean "part" in the everyday use of that word.