This is probably not something you need to know, but in case it ever is:

                            References from     References only in
                            anywhere            defining module
                        +--------------------+---------------------+
Uses _tls_get_addr()    |  global-dynamic    |  local-dynamic      |
                        +--------------------+---------------------+
Uses data relocations   |  initial-exec      |  local-exec         |
                        +--------------------+---------------------+

global-dynamic: most general, safest, the default.

local-dynamic: usable iff the TLV is defined in the same object as it is
referenced (eg, file scope, or protected or hidden visibility).

initial-exec: preallocated slots (finite)

local-exec: intersection of initial-exec and local-dynamic