Every glyph is a sign, but a glyph can also contain signs. Signs are carrying information to the reader. Take for example the type of glyph looking like a bird (left variant below). It occurs in many places and then mostly without any further signs. But there are cases when this bird clearly has been given a sign in the form of an 'undulating' wing (right below)

This strange wing surely must mean something, i.e. carry additional information.

Now - to complicate matters - what is a sign in one rongorongo text might not be so in another. Take for example:

Here we can see yet an additional sign, this time an eye whatever that means. This glyph has three signs: 1. The bird itself. 2. An undulating wing. 3. An eye (which is unusual). However the eye is not a sign in the text of tablet P (Large St Petersburg Tablet). For in this text the bird as a rule always has an eye, not unusual at all (example left below). Once though (right below) the eye is missing and that constitutes a sign in Large St Petersburg Tablet: