Clarification on addition boundries

Good evening everyone.

I was wondering if someone would be able to clarify something for me.

I'm currently a teaching assistant trainee for a small school in Staffordshire, England.

I can teach people all about biology, ICT and even German - Maths however is a different story :(

This term I am helping a year 4 class with math problems. I've been working through my lesson plan fairly easy, but tonight I have reached a small snag.

I've been working through guidelines such as counting back by 10,100, 1000. Recall Addidtion/Subtraction, etc etc.

One of them is "Add/subtract a pair of two-digit numbers (not crossing

10 or 100 boundary)"

Can someone clarify what is meant by this please? Obviosuly I know how to add/subtract two digit numbers, but what is this 10/100 boundry? Does this just mean I can not go below 10 and above 100, or am I missing something?

Any help would be greaty appreciated :)