At library level

This use_package_clause makes the content of a package directly visible. If applied on the library level, its scope is the complete library unit.

A use clause never hides anything that is already directly visible without it. All use clauses in effect in a certain scope act on the same level: Two homographs in different packages must still be named as selected components.

with Ada.Text_IO;   use Ada.Text_IO;

procedure Hello is
begin
   Put_Line("Hello, world!");
   New_Line;
   Put_Line("I am an Ada program with package use.");
end Hello;

If readability is your main concern then you should avoid this kind of use clause. Either make it more local or use a use_type_clause.

At declaration level

The effect is the same as above, but the scope is the directly enclosing declaration region.

with Ada.Text_IO;

procedure Hello is
   use Ada.Text_IO;
begin
   Put_Line("Hello, world!");
   New_Line;
   Put_Line("I am an Ada program with package use.");
end Hello;

For types

Use type clauses come in two versions.

use type Type_Name;

makes the operators of the type directly visible.

use all type Type_Name;

makes the primitive operations of the type directly visible.

Wikibook

• Ada Programming
• Ada Programming/Keywords

Ada Reference Manual

• 2.9: Reserved Words ^[Annotated]
• Annex P: (informative) Syntax Summary ^[Annotated]

• 8.3: Visibility ^[Annotated]
• 8.4: Use Clauses ^[Annotated]
• 13.3: Operational and Representation Attributes ^[Annotated]
• 13.4: Enumeration Representation Clauses ^[Annotated]
• 13.5.1: Record Representation Clauses ^[Annotated]

Ada Quality and Style Guide

• 3.1.3 Capitalization