.subnav is navigation that is typically used when on a dashboard type interface with another set of navigation above it. This helps distinguish navigation hierarchy.
<nav class="subnav" aria-label="Repository"><a class="subnav-item" href="#url" aria-current="page">Item 1</a><a class="subnav-item" href="#url">Item 2</a><a class="subnav-item" href="#url">Item 3</a></nav>
You can have subnav-search in the subnav bar.
<div class="subnav"><nav class="subnav-links" aria-label="Repository"><a class="subnav-item" href="#url" aria-current="page">Item 1</a><a class="subnav-item" href="#url">Item 2</a><a class="subnav-item" href="#url">Item 3</a></nav><div class="subnav-search float-left"><input type="search" name="name" class="form-control subnav-search-input" value="" aria-label="Search site" /><!-- <%= octicon "search", class: "subnav-search-icon" %> --><svg class="subnav-search-icon octicon octicon-search" width="16" height="16" viewBox="0 0 16 16" aria-hidden="true"> <path fill-rule="evenodd" d="M15.7 13.3l-3.81-3.83A5.93 5.93 0 0 0 13 6c0-3.31-2.69-6-6-6S1 2.69 1 6s2.69 6 6 6c1.3 0 2.48-.41 3.47-1.11l3.83 3.81c.19.2.45.3.7.3.25 0 .52-.09.7-.3a.996.996 0 0 0 0-1.41v.01zM7 10.7c-2.59 0-4.7-2.11-4.7-4.7 0-2.59 2.11-4.7 4.7-4.7 2.59 0 4.7 2.11 4.7 4.7 0 2.59-2.11 4.7-4.7 4.7z" /> </svg></div></div>
You can also use a subnav-search-context to display search help in a select menu.
<div class="subnav"><nav class="subnav-links"><a class="subnav-item" href="#url" aria-current="page">Item 1</a><a class="subnav-item" href="#url">Item 2</a><a class="subnav-item" href="#url">Item 3</a></nav></div><div class="subnav"><div class="float-left subnav-search-context"><button class="btn" type="button" name="button" aria-expanded="false" aria-haspopup="true">Filters<span class="dropdown-caret"></span></button></div><div class="subnav-search float-left"><input type="search" name="name" class="form-control subnav-search-input" value="" aria-label="Search site" /><!-- <%= octicon "search", class: "subnav-search-icon" %> --><svg class="subnav-search-icon octicon octicon-search" width="16" height="16" viewBox="0 0 16 16" aria-hidden="true"> <path fill-rule="evenodd" d="M15.7 13.3l-3.81-3.83A5.93 5.93 0 0 0 13 6c0-3.31-2.69-6-6-6S1 2.69 1 6s2.69 6 6 6c1.3 0 2.48-.41 3.47-1.11l3.83 3.81c.19.2.45.3.7.3.25 0 .52-.09.7-.3a.996.996 0 0 0 0-1.41v.01zM7 10.7c-2.59 0-4.7-2.11-4.7-4.7 0-2.59 2.11-4.7 4.7-4.7 2.59 0 4.7 2.11 4.7 4.7 0 2.59-2.11 4.7-4.7 4.7z" /> </svg></div></div>