Long texts are maintained for materials in material master in ECC system.
ON ECC R/3 side -
In the Basic Data1 tab for the material, these texts could be found by clicking the button as below -

The text is maintained in the text box control -

When the materials are replicated to SRM using material replication, these long texts are also replicated.
On SRM side -
In table COMC_PR_TEXT_M mapping of text objects and text IDs is maintained. Text Objects and Text IDs from R/3 side are mapped to text objects and Text IDs on SRM side in this table.
In STXH table, mapping of text object and text ID for the product GUID is maintained.
In table COMM_PRODUCT, we can find the GUID for the product number.
By passing parameters to FM 'READ_TEXT' we can get the long texts maintained for the product.

In the Tables section of the FM, 'Lines' parameter will return the long text maintained for the material.
On SRM side, using BBP_LONGTEXT_BADI, this material long text can be mapped to notes & attachments section long texts of documents if required.