Or you can save in AC10 as an Archive (*.PLA extension) so your placed objects will be there in AC11.
AC10 cannot open AC11 version files.
If you save as AC10 version in AC11 and used AC11 library parts in AC11, AC10 will not be able to read those library parts, because their library part version is higher than the one AC10 recognizes.
Loving Archicad since 1995 - Find Archicad Tips at x.com/laszlonagy
AMD Ryzen9 5900X CPU, 64 GB RAM 3600 MHz, Nvidia GTX 1060 6GB, 500 GB NVMe SSD
2x28" (2560x1440), Windows 10 PRO ENG, Ac20-Ac28