Difference between a single line and association line in UML

Question

I have the following class diagrams taken from a book
In Figure 6.4 there is a plain line between part and purchase order
line item. Whereas in figure 6.11 there is an arrow from
purchase order line item to part
What's the difference between these two ?

Answer 1

In Figure 6.4 there is a plain line between part and purchase order line item

in this figure the relation is (probably) navigable in the two directions, that means Part and Purchage Order Line Item reference/know each other

Typically the class Purchage Order Line Item has an attribute of type Part, and the class Part has an attribute of type Purchage Order Line Item (the multiplicities are not indicated, let say they are both 1 else they are collection of).

This is the case E-F in figure 11.29 Examples of navigable association-owned ends page 205 of formal/2017-12-05

in figure 6.11 there is an arrow from purchase order line item to part

In this diagram the relation is navigable in only one direction, Purchage Order Line Item reference/know the Part but Part does not know/reference the Purchage Order Line Item knowing/referencing it

Typically the class Purchage Order Line Item has an attribute of type Part, but the class Part does not have an attribute of type Purchage Order Line Item (the multiplicities are not indicated, let say they are both 1 else they are collection of).

This is the case I-J in figure 11.29 Examples of navigable association-owned ends page 205 of formal/2017-12-05