The phrase "side door" refers to a less common or unconventional way to achieve or gain access to something, especially when the usual or direct approach may not be available or effective. Full definition
This minivan offers seating configurations for seven or eight, as well as available dual power sliding side doors for optimal access. (toyotaofgreenwich.com)
The worst about this car is that it has a lot of little problems like the windows not rolling up sometimes or not being able to open the drivers side door from the inside. (autolist.com)
I left my passenger side door open and jumped in the driver's seat. (cleantechnica.com)