Published Dec 06, 2021
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Packages can support having multiple versions installed simultaneously. This is useful for libraries which may have changed interfaces between versions — for example, the gtk+ package can install both versions 2.24 and 3.6 in parallel. This feature is called slotting.
Most packages have no need for slotting. These packages specify SLOT="0" in the ebuilds. This is purely a convention; the package manager does not treat 0 any different from other slot values.
Portage permits at most one instance of a package installation per SLOT value. For example, say we have the following:
foo-1.1 with SLOT="1"foo-1.2 with SLOT="1"foo-2.0 with SLOT="2"foo-2.1 with SLOT="2"Then the user could have, say, foo-1.2 and foo-2.0 installed in parallel,
but not foo-1.1 and foo-1.2. Note that it is entirely possible that the user
may have foo-2.0 installed and no foo-1.x at all.
To DEPEND upon a package in a specific slot, refer to SLOT Dependencies.