soえるeru
1.

To add something supplementary to a primary item, enhancing or complementing its overall expressiveness or aesthetic value, as in attaching a note to a gift or adding colors to a dish.

2.

To place an additional item beside the main one, such as incorporating vegetables with meat in a meal or adding a musical chorus to an event for greater effect.

3.

To provide support or assistance by including someone to accompany another, which can be seen in the context of a courier bringing along a companion.

4.

To draw a figurative comparison by likening one thing to another, often used poetically or in literary contexts to evoke imagery or emotions.

Example:

彼は花にきれいなカードを添えて彼女に送った。

kakisoえるeru
1.

To include additional words or notes in a written document, such as adding personal remarks or comments to a letter or text that already exists.

2.

To augment an existing piece of writing or artwork with supplementary text, enhancing the original message or providing extra information.

3.

To make a minor addition to a completed piece, often for the purpose of clarification or further elaboration on a specific point.

Example:

一両日中に上京すると書き添えてあった。