Minecraft wiki entry for NBT_path_format, associated text: {}—Specifies the root tag
{foo:4.0f}—Specifies the root tag if the "foo" subtag is set to 4.0f
foo—Specifies the tag named "foo" under the root tag
foo.bar or foo{}.bar—Specifies foo's subtag named "bar".
foo.bar[0]—Specifies the first element of the list (or array) "bar"
foo.bar[-1]—Specifies the last element of the list (or array) "bar"
foo.bar[0]."A [crazy name]!"—Specifies the subtag named "A [crazy name]!" under that first element
foo.bar[0]."A [crazy name]!".baz—Specifies the subtag named "baz" under that crazily named tag
foo.bar[]—Specifies all elements of the list (or array) "bar"
foo.bar[].baz—Specifies the subtags named "baz" under all elements of the list "bar"
foo.bar[{baz:5b}]—Specifies all elements of the list "bar", in which the "baz" tag is set to 5b
foo{bar:"baz"}—Specifies the "foo" tag if its subtag "bar" has the value "baz"
foo{bar:"baz"}.bar—Specifies the "bar" tag if it matches the value "baz"