I hit my 10 year mark in the Army August 2018 and my reenlistment window opens in October 2018. Will I have to reenlist indefinite?

The indef mark, now called the Career Status Program, is at the 12 year mark.

To add to all the other comments, I want to clarify that you reenlist under the career status program ("indefinite reenlistment") once your contractual ETS is at 12 years or more years of Active Federal Service (AFS), not when you are actually past 12 years. For example, My BASD is January 2009. Right now my contractual ETS (May 2019) puts me at 10 years and 4 months AFS. with 10 months left until ETS, if I reenlist for 3 or more years, my AFS at ETS will be over 12 years, so my next reenlistment would be under the career status program.

Instead, lets say I reenlist for 2 years in December 2018, that would put my ETS at 11 years 11 months AFS. I could then reenlist again in September 2019 (15 month window from ETS) for, lets say, 2 more years, which would put me at 12 years 8 months AFS at ETS. When my reenlistment window opens again (June 2020), I would only be at 11 years 5 months AFS, but my ETS is over 12 years AFS, so I would have to reenlist for the career status program. Get it?

Those was the rule before I retired in 2005.