After I picked the date at Sterile Corridor required boarding bass to enter, then the bottom of screen said:
Special Instruction: Applicable only to those with BOARDING PASS the same day, or current access on the same day, i.e. flying domestic at IAH.
Does anyone know exactly what it means?
Thanks.

I know this message doesn't help the OP as it's been months, but the special instruction indicates a boarding pass is needed to access this particular office as it's past security.

Long, but detailed
 

Thought I should share my detailed experience with my Global Entry (GE) application process from start to finish at IAH.

Important Dates

• Dec. 7, 2019: Applied for GE, Paid the Fee & Submitted my Application

• Dec. 12, 2019: Conditionally Approved
  • I will admit I am young, with no criminal history, and have been previously issued a federal ID (not from GE or other similar programs) so this may have been the reason for the quick turnaround, but who really knows...

• Dec. 13, 2019: Interviewed at IAH Terminal E in Secure Area
  • There're apparently 2 enrollment centers at IAH. I went to the one in SECURE area of Terminal E, which REQUIRES a valid boarding pass. This will be helpful in my next point.
  • If you're anyone like me, I had no plans to fly out of IAH any time soon especially for the sole purpose of attending this interview. Needless say, after doing my due diligence of looking up which airline had the most lenient refund policy for its passengers. I ended up "booking" (emphases on booking...) a flight from IAH and used that boarding pass to get through security and into the secure area of Terminal E. Note that it does NOT have to be an international flight, but be aware that some airlines will only issue a full refund back to the original method of payment only if you purchase a certain type of ticket. I purchased a first class ticket and ended up getting a full refund back to my card soon after I canceled my flight within the timeframe as outlined by the airline's refund policy.
  • Note that scheduling an interview at IAH or any major metropolitan city/airport for that matter may take months to secure a date/time. From personal experience, I strongly recommend that you check the availability more than periodically. To put it into perspective, I was checking every 20-30 minutes the day I got conditionally approved and finally secured an interview for the following day.
  • ALSO, this particular enrollment center DOES accept walk-in interviews. HOWEVER, please note that those with an actual appointment WILL take priority, which means you may or may not get interviewed that day. The only way walk-in's will be accepted is if only someone cancels/ is late to their scheduled appointment (I know this because that's what the enrollment center officer told the person in front of me who didn't have an appointment that day).

• Dec. 16, 2019: Final Approval - YAY :)

Q&A's

What was the interview process like?

• It was quick. Likely less than 10 minutes. The enrollment center officer just verified my personal information on my application against my passport and drivers license. Took my fingerprints and photo. Asked me to confirm my name, date of birth, address, eMail, things you'd expect a government official to normally ask. Wouldn't really call it an "interview". The only interview question I was asked was why I wanted GE. Other than that it was just a normal conversation about life. The person was amicable though I'm sure they rotate so every experience may vary depending on who is assigned to do the job that date/time.

Well, I hope somebody finds this helpful and I wish you all the best of luck on your GE application process!