Let $ABCD$ be a rectangle. Determine the set of all points $P$ from the region between the parallel lines $AB$ and $CD$ such that $\angle APB=\angle CPD$.
To determine the set of all points \( P \) from the region between the parallel lines \( AB \) and \( CD \) such that \( \angle APB = \angle CPD \), we need to analyze the geometric properties of the rectangle and the angles formed by the points.

1. **Identify the Geometry of the Rectangle:**
   Let \( A = (0, 0) \), \( B = (a, 0) \), \( C = (a, b) \), and \( D = (0, b) \). The lines \( AB \) and \( CD \) are parallel and horizontal, while \( AD \) and \( BC \) are parallel and vertical.

2. **Position of Point \( P \):**
   Let \( P \) be a point in the region between the lines \( AB \) and \( CD \). We need to find the condition under which \( \angle APB = \angle CPD \).

3. **Angle Condition Analysis:**
   Consider the angles \( \angle APB \) and \( \angle CPD \). For these angles to be equal, the point \( P \) must be positioned such that the triangles \( \triangle APB \) and \( \triangle CPD \) are similar or congruent in a specific manner.

4. **Symmetry and Reflection:**
   Notice that if \( P \) is the midpoint of the line segment joining \( AB \) and \( CD \), then the angles \( \angle APB \) and \( \angle CPD \) will be equal due to symmetry. This is because the diagonals of a rectangle bisect each other and create congruent triangles.

5. **Midline of the Rectangle:**
   The set of all points \( P \) that satisfy \( \angle APB = \angle CPD \) will lie on the line that is equidistant from \( AB \) and \( CD \). This line is the horizontal line passing through the midpoint of the rectangle.

6. **Equation of the Midline:**
   The midpoint of the rectangle is at \( \left(\frac{a}{2}, \frac{b}{2}\right) \). The line passing through this point and parallel to \( AB \) and \( CD \) is given by \( y = \frac{b}{2} \).

Therefore, the set of all points \( P \) such that \( \angle APB = \angle CPD \) is the line \( y = \frac{b}{2} \).

\[
\boxed{y = \frac{b}{2}}
\]