So, I'm new to Dark Heresy, and planning out a character for an upcoming campaign. Can I spend xp in a previous rank even though I am in the next rank up?

To try and clarify, planning my chosen skills and talents for my Scholar Medicae rank, can I pick up more skills from the Scholar Materium rank which is the rank prior?

You absolutely can!

There are some exceptions to this rule, though - for example, "Sound Constitution" can still only be taken X times per Rank, depending on what it says in your advancement scheme. Basically, regard your Rank table as "unlocks" that you still have to purchase, but you can do so at any time you want.

You absolutely can!

There are some exceptions to this rule, though - for example, "Sound Constitution" can still only be taken X times per Rank, depending on what it says in your advancement scheme. Basically, regard your Rank table as "unlocks" that you still have to purchase, but you can do so at any time you want.

That's not the exception, it's the rule.

For example, if the Advancement Schemes you have unlocked only gives you access to Awareness +10, you cannot rank up the Awareness Skill to +20.

Additionally, if none of your unlocked Advancement Schemes gives you access to Awareness, you'll have to negotiate with your GM to buy Awareness as an Elite Advance before you can buy it, even if one of your unlocked Advance Schemes gives you access to Awareness +10.

Basically, regard your Rank table as "unlocks" that you still have to purchase, but you can do so at any time you want.

That's what I thought, but I wasn't 100% sure. Thanks for the confirmation though!