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IUSE flags are simply the list of available and default USE
flags for a package as defined by the Ebuild maintainer.


### Definition

IUSE is defined from a developer's perspective in 
[`man 5 ebuild`](https://dev.gentoo.org/~zmedico/portage/doc/man/ebuild.5.html)
(not `man ebuild`!):

>**IUSE**
>
>    This should be a list of any and all USE flags that are leveraged
within your build script. The only USE flags that should not be listed here
are arch related flags (see **KEYWORDS**). Beginning with **EAPI 1**, it
is possible to prefix flags with + or - in order to create default settings
that respectively enable or disable the corresponding USE flags. For details
about **USE** flag stacking order, refer to the **USE_ORDER** variable in
make.conf(5). Given the default **USE_ORDER** setting, negative IUSE default
settings are effective only for negation of repo-level USE settings, since
profile and user configuration settings override them.

There is also a quick hint about [IUSE defaults in the Dev-manual](https://devmanual.gentoo.org/general-concepts/use-flags/index.html#iuse-defaults).

### Example

In Ulogd's Ebuild, the `IUSE` flags line is:

```
$ grep IUSE /var/db/repos/gentoo/app-admin/ulogd/*.ebuild
IUSE="dbi doc json mysql nfacct +nfct +nflog pcap postgres selinux sqlite ulog"
```

Those are exactly the USE flags available to emerge Ulogd with their 
respective defaults:

```
$ emerge -vp ulogd
[ebuild  N     ] app-admin/ulogd-2.0.7-r2::gentoo  USE="nfct nflog -dbi -doc 
-json -mysql -nfacct -pcap -postgres (-selinux) -sqlite -ulog"
```