Channels (Hubzilla & (streams))
Most Fediverse projects only offer one identity per user account.
Hubzilla, the Fediverse application commonly referred to as (streams) and Forte go further than this: An account can contain one or multiple independent channels. Each one of these channels is a separate identity that can be compared to one Friendica account with variable roles and access permission settings.
Channels as separate identities
Similar to Friendica, Hubzilla still has multiple profiles per channel. But these different profiles only serve to show different aspects of the same identity to specific users. For example, you may have only basic information about yourself in your default public profile, a separate profile for your work colleagues, another separate profile for your close friends etc. But it's still the same identity, and different profiles can't conceal that. (streams) and Forte don't offer this feature.
Another channel, in contrast, is actually another fully separate identity, much like having another account on e.g. Mastodon. Other users can't tell which channels belong to the same account because the account underneath the channels doesn't identify itself anywhere, only the channels do. Thus, it is not necessary to have multiple accounts on the same server to maintain multiple separate online identities.
Each channel has its own separate profile(s), its own separate settings, its own separate posts, its own separate content altogether and its own connections. Other users can't follow your whole account, only the channels on it individually. The only common settings for all channels are those for the account itself.
Channels for separate purposes
Since Hubzilla, (streams) and Forte channels can be assigned various different roles with different access permission settings, another use-case for multiple channels on one account is to have an extra channel for a special purpose.
For example, next to your personal channel, you could run a forum which is nothing else than another channel with a specific role. Or you could have a private Hubzilla channel which you only use as a feed aggregator. Or if you want to start a blog, you could decide to keep it separate from your personal channel and create a new channel for it. Or you could make a Hubzilla channel for a website.
Handling channels
The advantage of having multiple channels on one account over having multiple separate accounts on the same server is that you don't have to log out and back in to switch to another identity. By default, all you have to do is navigate to the channel manager and choose another channel. You can have your channels listed in your profile menu and make them accessible more easily. On Hubzilla, this entire feature is optional and off by default; (streams) and Forte let you select which of your channel to add to the menu. Only you yourself can see these channels listed, no-one else who visits your channel can.
One channel is always the default channel. By default, it's the channel that is automatically created along with a new account. This can be changed in the channel manager where channels can also be created or deleted. Be careful deleting channels: At least Hubzilla never makes the IDs of deleted channels available again unless the hub admin manually removes them from the hub database.
Channels and nomadic identity
The multiple-channels-per-account model has another advantage: It greatly facilitated the introduction of nomadic identity. A channel basically puts your identity into a "container". What nomadic identity does is copy the whole container with everything in it, posts, settings, connections, files etc., to another account on another server. The clone can optionally be automatically kept in sync with the original, i.e. whatever happens on the original is automatically mirrored to the clone and vice versa. At the same time, the clone keeps the ID of the original. It would be a lot more difficult to pry certain data directly from one account, copy it directly to another account, keep it in sync and keep the copied data on the second account assigned to the ID of the first account.
By the way, each channel on an account can be cloned independently from the other channels. You don't have to clone all your channels to the same backup account. In fact, you don't have to clone all your channels at once. And you can have uncloned channels, main instances of cloned channels and/or clones of channels with main instances elsewhere on the same account.