Roy,
In surveyor parlance, Tangent are straight line segment along an alignment.
PC stands for point of curvature, (beginning of Arc) although nowadays we use TC for Tangent to Curve.
PT is point of tangency that is end of the arc, nowadays CT for Curve to Tangent.
PI stand for point of intersection of two tangent.
Similarly you also have
TS Tangent to Spiral
SC Spiral to Curve
CS Curve to Spiral
ST Spiral to Tangent
ymg