DynamoRIO core documentation

I have two simple questions about the coding style.

1. There are many functions like "os_set_dr_tls_base" and structures like "os_local_state_t" starting with an "os" prefix. What does this prefix stand for? My guess is that these functions and structures are shared by all Unix systems, including Macros, Linux, and Unix. Am I right?

2. A similar question is about the "alt" phrase in "priv_alt_tls_base" and "app_alt_tls_base"? Does it stand for "alternative"? Does this phrase imply that these TLS-segments would be installed and uninstalled when control-flow transfers from DynamoRIO to the target application, and vice versa?

Another question is about the "private loader". Does it work like ld.so to provide functionalities for loading a program and all dynamically linked libraries? What does its role in DynamoRIO? Could you give a brief explanation?

Thanks!