What does the grey portion of the experience bar denote?

The experience bar sometimes has a grey portion after the already gained experience (circled):

As stated, this grey portion isn't always present. What does the grey portion indicate?

Solution 1:

Based on this Reddit thread and this guide, the grey bar represents how much Rested XP you have accumulated. Rested XP can be obtained by logging out while in a town/settlement (the Reddit threads stated it starts accumulating after 8 hours of inactivity, but not sure if that’s accurate).

Rested XP is essentially bonus XP you can take advantage of towards your character level; it does not affect crafting or trade skills. When you have Rested XP, any task that provides character experience is doubled until the Rested Xp is consumed.