The system provides lots of small icons—representing common tasks and types of content—for use in navigation bars and tab bars. It’s a good idea to use these built-in icons as much as possible because they're familiar to people.
Use system icons as intended. Every system-provided image has a specific, well-known meaning. To avoid confusing users, it’s essential that each image be used in accordance with its meaning and recommended usage.
Design a custom icon if you can’t find a system-provided one that meets your needs. It’s better to design your own than to misuse a system-provided image. See Custom Icons.
Navigation Bar Icons
TIP You can use text instead of icons to represent items in a navigation bar. You can also use a fixed space element to provide padding between navigation and toolbar icons.