Sometimes an Invitation can slide over into a Challenge. At a fundamental level, the difference is in our internal reaction to the item. For some going to a party is an invitation, for some it is a challenge. For some a 3 minute journalling practice is an invitation, for others a challenge. If it is easy or requires overcoming minimal barriers to engage with, it’s likely an invitation. When engaging with something requires overcoming a lot of internal resistance (of whatever nature), that invitation has become a challenge.
And sometimes the challenge is truly a challenge for most everyone (requiring a lot of effort, challenging our own perceptions, our state of being, etc.).
Challenges are a choice, not a requirement. But there may be the odd time that a challenge becomes a prerequisite to another map or practice. This is not done to force anyone into a challenge but is a natural and necessary consequence of the reality that there are levels to the practices in Terra Arcanum, that in order to be successful at a more complicated thing, we must first be proficient at other things.
For more such thoughts and clarifications on challenges: