understanding datastore in bitbake

I am new to bitbake. I am going through the user manual for learning and I got a doubt in the section quoted below > ### 3.5.3. Passing Information Into the Build Task Environment > > When running a task, BitBake tightly controls the execution > environment of the build tasks to make sure unwanted contamination > from the build machine cannot influence the build. Consequently, if > you do want something to get passed into the build task environment, > you must take these two steps: > > 1. Tell BitBake to load what you want from the environment into the datastore. You can do so through the BB_ENV_EXTRAWHITE variable. For > example, assume you want to prevent the build system from accessing > your $HOME/.ccache directory. The following command tells BitBake to > load CCACHE_DIR from the environment into the datastore: > > export BB_ENV_EXTRAWHITE="$BB_ENV_EXTRAWHITE CCACHE_DIR" > > > 2. Tell BitBake to export what you have loaded into the datastore to the task environment of every running task. Loading something from > the environment into the datastore (previous step) only makes it > available in the datastore. To export it to the task environment of > every running task, use a command similar to the following in your > local configuration file local.conf or your distribution configuration > file: > > export CCACHE_DIR It says from external environment i have to load necessary values to datastore and from there export to task environment. As per my understanding it is something like from my PC i have load in necessary data to a structure called datastore and that should be loaded in bitbake environment for it to get processed by bitbake. But i am not able to get a clear overview. Can you explain me (if possible with example) what are external environment variables, internal environment variable and datastore?