原生覆盖工具在源代码的每一行插入“bump”调用，记录模块、函数、arity、子句号和行号：

bump_call(Vars, Line) ->
    A = erl_anno:new(0),
    {call,A,{remote,A,{atom,A,ets},{atom,A,update_counter}},
     [{atom,A,?COVER_TABLE},
      {tuple,A,[{atom,A,?BUMP_REC_NAME},
                {atom,A,Vars#vars.module},
                {atom,A,Vars#vars.function},
                {integer,A,Vars#vars.arity},
                {integer,A,Vars#vars.clause},
                {integer,A,Line}]},
      {integer,A,1}]}.

(from覆盖
上述函数插入的代码为：

ets:update_counter(?COVER_TABLE,
  {?BUMP_REC_NAME, Module, Function, Arity, Clause, Line}, 1)

也就是说，将所讨论的给定模块/函数/行的条目递增1。在所有测试完成后，cover将使用此表中的数据，并显示给定行的执行次数。
正如the cover documentation中所提到的，你可以获得模块、函数、函数子句和行的覆盖率。看起来ExCoveralls只在它的报告中使用行覆盖率，但没有理由它不能完成所有四种类型的覆盖率。
不支持分支覆盖范围。似乎支持分支覆盖范围需要扩展“bump”记录并更新cover.erl以记录该信息。在有人执行此操作之前，仅当分支出现在不同行上时，覆盖范围信息才是准确的。例如：

case always_false() of
    true ->
        %% this line shows up as not covered
        do_something();
    false ->
        ok
end.

%% this line shows up as covered, even though do_something is never called
always_false() andalso do_something()

To add to @legoscia excellent response, I also want to clarify why cover does not do statements evaluation. According to this discussion in the official forum:
https://elixirforum.com/t/code-coverage-tools-for-elixir/18102/10
The code is first compiled into erlang and then from erlang into a modified binary file (but no .beam file is created) that is automatically loaded into memory and executed.
Because of the way erlang code works, a single statement can have several instructions:
and single line can result in multiple VM “statements”, for example:

Integer.to_string(a + 1)

Will result with 2 instructions:

{line,[{location,"lib/tasks.ex",6}]}.
{gc_bif,'+',{f,0},1,[{x,0},{integer,1}],{x,0}}.
{line,[{location,"lib/tasks.ex",6}]}.
{call_ext_only,1,{extfunc,erlang,integer_to_binary,1}}.

Therefore it is rather tricky for an automatic analysis tool to provide statement coverage because it is hard to match statements to instructions, especially as in theory a compiler is free to reorder commands as it pleases as long as the result is the same.