In IS-IS, Level 1 routes are usually contained within the same area and Level 2 routes are used to interconnect different areas. By default, routes from Level 1 are redistributed into Level 2, and vice versa, to ensure reachability between areas.
Reference:
Juniper Networks Technical Documentation on IS-IS
By default, IS-IS protocol leaks routing information from a Level 1 area to a Level 2 area. However, to leak routing information from a Level 2 area to a Level 1 area, an export policy must be explicitly configured.